Energy efficient polyolefin process

ABSTRACT

A manufacturing process for producing polyolefin, having a feed system, a reactor system including at least one polymerization reactor, a diluent/monomer recovery system, a fractionation system, and an extrusion/loadout system having an extruder. The manufacturing process is configured to consume less than 325 kilowatt-hours of electricity per metric ton of polyolefin produced.

BACKGROUND OF THE INVENTION

1. Related Applications

This application claims priority to U.S. Application Ser. No. 60/604,948filed on Aug. 27, 2004.

2. Field of the Invention

The present invention relates generally to polyolefin production and,more specifically, to techniques that increase the energy efficiency ofpolyolefin production processes.

3. Description of the Related Art

This section is intended to introduce the reader to aspects of art thatmay be related to aspects of the present invention, which are describedand/or claimed below. This discussion is believed to be helpful inproviding the reader with background information to facilitate a betterunderstanding of the various aspects of the present invention.Accordingly, it should be understood that these statements are to beread in this light, and not as admissions of prior art.

As chemical and petrochemical technologies have advanced, the productsof these technologies have become increasingly prevalent in society. Inparticular, as techniques for bonding simple molecular building blocksinto longer chains (or polymers) have advanced, the polymer products,typically in the form of various plastics, have been increasinglyincorporated into various everyday items. For example, polyolefinpolymers, such as polyethylene, polypropylene, and their copolymers, areused for retail and pharmaceutical packaging, food and beveragepackaging (such as juice and soda bottles), household containers (suchas pails and boxes), household items (such as appliances, furniture,carpeting, and toys), automobile components, pipes, conduits, andvarious industrial products.

Specific types of polyolefins, such as high-density polyethylene (HDPE),have particular applications in the manufacture of blow-molded andinjection-molded goods, such as food and beverage containers, film, andplastic pipe. Other types of polyolefins, such as low-densitypolyethylene (LDPE), linear low-density polyethylene (LLDPE), isotacticpolypropylene (iPP), and syndiotactic polypropylene (sPP) are alsosuited for similar applications. The mechanical requirements of theapplication, such as tensile strength and density, and/or the chemicalrequirements, such thermal stability, molecular weight, and chemicalreactivity, typically determine what type of polyolefin is suitable.

One benefit of polyolefin construction, as may be deduced from the listof uses above, is that it is generally non-reactive with goods orproducts with which it is in contact. This allows polyolefin products tobe used in residential, commercial, and industrial contexts, includingfood and beverage storage and transportation, consumer electronics,agriculture, shipping, and vehicular construction. The wide variety ofresidential, commercial and industrial uses for polyolefins hastranslated into a substantial demand for raw polyolefin which can beextruded, injected, blown or otherwise formed into a final consumableproduct or component.

To satisfy this demand, various processes exist by which olefins may bepolymerized to form polyolefins. Typically, these processes areperformed or near at petrochemical facilities, which have ready accessto the short-chain olefin molecules (monomers and comonomers) such asethylene, propylene, butene, pentene, hexene, octene, decene, and otherbuilding blocks of the much longer polyolefin polymers. These monomersand comonomers may be polymerized in a liquid-phase polymerizationreactor and/or gas-phase polymerization reactor to form a productcomprising polymer (polyolefin) solid particulates, typically calledfluff or granules. The fluff may possess one or more melt, physical,rheological, and/or mechanical properties of interest, such as density,melt index (MI), melt flow rate (MFR), copolymer content, comonomercontent, modulus, and crystallinity. The reaction conditions within thereactor, such as temperature, pressure, chemical concentrations, polymerproduction rate, and so forth, may be selected to achieve the desiredfluff properties.

In addition to the one or more olefin monomers, a catalyst forfacilitating the polymerization of the monomers may be added to thereactor. For example, the catalyst may be a particle added via a reactorfeed stream and, once added, suspended in the fluid medium within thereactor. An example of such a catalyst is a chromium oxide containinghexavalent chromium on a silica support. Further, a diluent may beintroduced into the reactor. The diluent may be an inert hydrocarbon,such as isobutane, propane, n-pentane, i-pentane, neopentane, andn-hexane, which is liquid at reaction conditions. However, somepolymerization processes may not employ a separate diluent, such as inthe case of selected examples of polypropylene production where thepropylene monomer itself acts as the diluent.

The discharge of the reactor typically includes the polymer fluff aswell as non-polymer components, such as unreacted olefin monomer (andcomonomer), diluent, and so forth. In the case of polyethyleneproduction, the non-polymer components typically comprise primarilydiluent, such as isobutane, having a small amount of unreacted ethylene(e.g., 5 wt. %). This discharge stream is generally processed, such asby a diluent/monomer recovery system, to separate the non-polymercomponents from the polymer fluff. The recovered diluent, unreactedmonomer, and other non-polymer components from the recovery system maybe treated, such as by treatment beds and/or a fractionation system, andultimately returned as purified or treated feed to the reactor. Some ofthe components may be flared or returned to the supplier, such as to anolefin manufacturing plant or petroleum refinery. As for the recoveredpolymer (solids), the polymer may be treated to deactivate residualcatalyst, remove entrained hydrocarbons, dry the polymer, and pelletizethe polymer in an extruder, and so forth, before the polymer is sent tocustomer.

The competitive business of polyolefin production continuously drivesmanufacturers to improve their processes in order to lower productioncosts. In an industry where billions of pounds of polyolefin product areproduced per year, small incremental improvements, for example, incatalyst activity, monomer yield, energy efficiency, diluent recovery,and so forth, can generate significant cost savings in the manufactureof polyolefins. Fortunately, technological advances over the years inraw materials, equipment design and operation, and the like, haveprovided great strides in reducing the capital, operating, and fixedcosts of polyolefin manufacturing systems. For example, catalystresearch has produced commercial catalysts with activity values that areorders of magnitudes higher than those of two to three decades ago,resulting in a striking reduction in the amount of catalyst consumed perpound of polymer produced, and also reducing the amount of downstreamprocessing (and equipment) used to deactivate and/or remove residualcatalyst in the polymer product. Further, advances in equipment designand operation have also increased diluent recovery so that less diluentmake-up is utilized. Technological advances have also improved monomeryield, which is a measure of the conversion of monomer, such as ethyleneor propylene, to a polymer or polyolefin, such as polyethylene orpolypropylene. Additionally, advances have also increased energyefficiency in polyolefin manufacturing.

In general, the production of polyolefin is an energy-intensive process,consuming electricity, steam, fuel gas, and so on. A common way tomeasure energy consumption for a given technology or manufacturingprocess is in units of energy per mass of polyolefin produced. In atypical polyethylene plant, electrical consumption may be 460kilowatt-hour per metric ton of polyethylene (kWh/ton PE) and higher.With steam, the consumption may be higher than 8 kilograms per metricton of polyethylene (kg/ton PE) (an equivalent 320 kWh/ton PE).Likewise, fuel gas consumption for a typical polyolefin plant, such as apolyethylene plant, may be 8 kg/ton or higher. Again, such energyconsumption generally contributes significant cost to the production ofpolyolefins, as well as to the downstream products constructed ofpolyolefins distributed to the customer.

A variety of equipment and operations within a polyolefin manufacturingprocess may consume energy. Noteworthy consumers of electricity within apolyolefin plant, for example, may include the pumps that circulate theliquid reaction mixture in the polymerization reactors (e.g., loopslurry reactors), the pumps that circulate the cooling medium (e.g.,treated water) through the polymerization reactor jackets, thecompressors that pressurize and return recycled diluent (and/or monomer)to the polymerization reactor, the blowers used to convey fluff andpellets, and the extruders that convert the polyolefin fluff topolyolefin pellets. Significant users of steam in a typical polyolefinplant may include heaters that flash liquid in the effluent of thepolymerization reactor, and fractionation columns that process recovereddiluent and/or monomer. Relatively large consumers of fuel gas mayinclude activation processes (which may utilize high heat) of thepolymerization catalyst, and operations that maintain adequatecombustible content in the plant flare header (in the feed to theflare). In general, extensive energy is required to polymerize themonomer and comonomer to polyolefin fluff, to process recycled effluentfrom the reactor, and to convert the polyolefin fluff to pellets.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the invention may become apparent upon reading thefollowing detailed description and upon reference to the drawings inwhich:

FIG. 1 is a block flow diagram depicting an exemplary polyolefinmanufacturing system for producing polyolefins in accordance with oneembodiment of the present techniques;

FIG. 2 is a process flow diagram of an exemplary feed system of thepolyolefin manufacturing system of FIG. 1 in accordance with oneembodiment of the present techniques;

FIG. 3 is a process flow diagram of an exemplary treater regenerationcircuit in accordance with one embodiment of the present techniques;

FIG. 4 is a block diagram of an exemplary method for regenerating atreater in accordance with one embodiment of the present techniques;

FIG. 5 is a process flow diagram of an exemplary catalyst preparationsystem of the feed system of FIG. 2 in accordance with one embodiment ofthe present techniques;

FIG. 6 is a process flow diagram of an exemplary catalyst activationsystem in accordance with one embodiment of the present techniques;

FIG. 7 is a process flow diagram of an exemplary reactor system and adiluent/monomer recovery system of the polyolefin manufacturing systemof FIG. 1 in accordance with one embodiment of the present techniques;

FIG. 8 is a diagrammatical representation of the exemplarypolymerization reactor of FIG. 7 showing the flow of cooling mediumthrough the reactor jackets in accordance with one embodiment of thepresent techniques;

FIG. 9 is a process flow diagram of an exemplary coolant system used inthe temperature control of the polymerization reactor of FIG. 8 inaccordance with one embodiment of the present techniques;

FIG. 10 is a diagrammatical representation of an exemplary continuoustakeoff discharge of the polymerization reactor of FIG. 7 in accordancewith one embodiment of the present techniques;

FIG. 11 is a cross section along line 11-11 of FIG. 10 showing a ramvalve arrangement in the continuous take off discharge assembly inaccordance with one embodiment of the present techniques;

FIG. 12 is a diagrammatical representation of a tangential location forthe continuous take off assembly in accordance with one embodiment ofthe present techniques;

FIG. 13 is a process flow diagram of the fractionation system of FIG. 1in accordance with one embodiment of the present techniques; and

FIG. 14 is a process flow diagram of the extrusion/loadout system ofFIG. 1 in accordance with one embodiment of the present techniques.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

One or more specific embodiments of the present invention will bedescribed below. In an effort to provide a concise description of theseembodiments, not all features of an actual implementation are describedin the specification. It should be appreciated that in the developmentof any such actual implementation, as in any engineering or designproject, numerous implementation-specific decisions must be made toachieve the developers' specific goals, such as compliance withsystem-related and business-related constraints, which may vary from oneimplementation to another. Moreover, it should be appreciated that sucha development effort might be complex and time consuming, but wouldnevertheless be a routine undertaking of design, fabrication, andmanufacture for those of ordinary skill having the benefit of thisdisclosure.

The present techniques increase energy efficiency in the manufacture ofpolyolefin, such as polyethylene or polypropylene. Such reductions inenergy requirements may be accomplished, for example, by reducingprocessing steps, eliminating equipment, and using other techniques forimproving efficiency. As discussed throughout the disclosure, examplesof techniques that may enhance energy efficiency include reduction inthe fractionation and compression of recycle diluent and othercomponents, the combination of vessels into dual uses, and so forth. Ingeneral, such reduction of the energy required to produce a pound ofpolyolefin is beneficial for production facilities throughout the world,with increased benefits in parts of the world where energy is relativelyexpensive.

To facilitate discussion of the present techniques, the disclosure ispresented in sections. Section I introduces examples of energy-efficienttechniques in the polyolefin manufacturing process. Section II providesan overview of an exemplary polyolefin production process. Section IIIdiscusses the feed system for the polymerization reactor. Section IVdiscusses the polymerization reactor system. Section V discusses thediluent/monomer recovery system that may recover diluent and unreactedmonomer from the effluent that discharges from the polymerizationreactor. Section VI focuses on the cooling of the polymerizationreactor. Section VII discussed the use of guide vanes in the reactorcirculation pump to improve pumping efficiency and to increasepolyolefin production rate. Section VIII discusses the implementation ofa continuous takeoff for the polymerization reactor discharge. SectionIX discusses an exemplary fractionation system that may process a largeportion of the recycled diluent, or that may only treat a small portionto make available olefin-free diluent used in catalyst preparation.Section X discusses the extrusion/loadout system that converts the rawpolyolefin particles to polyolefin pellets for distribution to thecustomer. Lastly, Section XI summarizes exemplary energy-efficienttechniques. It should be noted that examples of the present techniquesthat provide for reducing energy consumption in the production ofpolyolefin are discussed throughout the disclosure. Furthermore, thoughthe discussion at times may focus on the production of polyethylene andits copolymer, the disclosed techniques afford benefits in increasedenergy-efficiency in the production of other polyolefins, such aspolypropylene, polybutylene, and so on. Finally, it should be apparentthat the various techniques may be implemented in a multiplicity ofcombinations.

I. Introduction to Examples of Energy-Efficient Techniques

The present techniques provide for increased energy-efficiency in a widerange of operations in the polyolefin manufacturing process. Suchoperation included, for example, the reactor feed system, reactorsystem, diluent/monomer recovery system, fractionation system, andextrusion/loadout system. These energy-saving techniques are introducedin this Section and discussed in detail in Sections below.

A. Reactor Feed System

In the polymerization reactor feed system, for example, a mass flowmeter, instead of the conventional orifice plate meter, is used tomeasure flow of monomer, eliminating preheating of the monomer. Further,a larger catalyst activator may be employed, reducing the amount of fuelgas consumed to activate the polymerization catalyst fed to the reactor.Additionally, the number of treaters that remove catalyst poisons fromthe reactor feed streams may be reduced, providing for more efficientregeneration of the treaters. Moreover, an improved technique ofregenerating the treaters reduces the amount of inert components (e.g.,nitrogen) discharged to the flare header. As a result, less fuel gas(e.g., natural gas) is injected into the flare header to maintain anappropriate combustible content of the feed to the flare.

B. Polymerization Reactor System

In the reactor system itself, a continuous take off (CTO) of thepolyolefin slurry discharged from the polymerization reactor, in lieu ofthe conventional intermittent discharge employing settling legs, allowsthe reactor to operate with a higher solids concentration. A largerconcentration of polyolefin in the reactor may afford a greaterproduction rate of the polyolefin. In general, increases in productionrate may reduce the normalized consumption of energy, in part, byspreading fixed energy costs over more pounds of polyolefin produced.Another benefit of the CTO discharge is that more polyolefin fineparticles (relatively small particles) or “fines” may be removed fromthe reactor than with the conventional settling-leg configuration.Therefore, with less surface area of particles in the reactor, the fluidmixture operates at a lower viscosity facilitating circulation of thereactor contents and lowering horsepower requirements of the reactorcirculation pump. Moreover, larger production rates can be achieved, forexample, with 2-3 CTO's in normal operation for a single reactor, with1-2 CTO's on standby. Further, each CTO may have a dedicated flash lineheater. However, with the conventional settling leg configuration, asmany as 12-14 would be used in operation to obtain the same productionrates, with each settling leg having a dedicated flash line heater.Thus, with the CTO's, gives higher production rates of the polyolefin,but with lower steam usage.

Additionally, a liquid phase reactor, such as a loop slurry reactor, maybe constructed of a material (e.g., high-strength aluminum) havinghigher strength and/or thermal conductivity than the steel materialstraditionally utilized in fabrication of the loop slurry reactor. Suchnewer high-strength materials may provide for improved thinner reactorwalls, increased heat-transfer through the walls, and a larger diameterof the loop reactor, permitting a higher polyolefin production rate. Yetanother example in the reactor system is the use of guide vanes in thereactor circulation pump, providing for increased pumping efficiency(reduced electrical consumption) and increased polyolefin productionrate. Lastly, another example is a technique that specifies a greaterincrease in the temperature (e.g., from the traditional 10° F. to thepresent 15-45° F. and higher) of the coolant flowing through the reactorjackets. Such increased temperature difference between the coolantsupply and return imparts substantially the same heat removal capabilitybut at lower flow rates of coolant. Therefore, the coolant circulatingpump may be sized smaller, requiring less horsepower.

C. Diluent/Monomer Recovery System

In the diluent/monomer recovery system that processes the effluentdischarged from the polymerization reactor, savings in electricity maybe accomplished by eliminating a low-pressure flash of the diluent andthe associated recycle compression. Further savings may be acquired byeliminating the purge column that removes residual hydrocarbon from thepolyolefin fluff particles. The hydrocarbon removal operation insteadmay be performed at the downstream extruder feed tank in theextrusion/loadout system. This improvement allows for utilization of theprocess pressure in an upstream flash chamber, instead of a blowerconveying system which consumes electricity, to transport the polyolefinparticles to the extruder feed tank. The improvement also provides forwarmer polyolefin fluff particles fed to the downstream extruder, thusreducing the energy load on the extruder.

D. Extrusion/Loadout System

Furthermore the number of polyolefin fluff silos intermediate thediluent/monomer recovery system and the extrusion/loadout system may bereduced in number via, in part, improved operation of the upstreampolymerization reactor and the downstream extruder. Such reduction insilos or storage vessels may reduce the number of associated blowers andtheir electrical consumption. In the extrusion/loadout system,electricity may be saved via use of a pellet water pump to transportpolyolefin pellets discharged from the extruder/pelletizer to the pelletsilos instead of the conventional blower conveying package. Indeed, thehorsepower requirement for the pellet water pump typically is an orderof magnitude lower than that of a pneumatic conveying blower.

E. Fractionation System

In the fractionation system that processes the recovered unreactedmonomer and diluent from the polymerization reactor and diluent/monomerrecovery system, steam usage may be reduced by as much as 90 percent.Such reduction may be afforded by direct recycling of the diluent andmonomer to the polymerization reactor and bypassing the fractionationsystem, thus allowing for smaller fractionation columns and smallerassociated steam reboiler heat-exchangers.

F. Representation of Exemplary Results

Table 1 below shows an example of improved energy-efficiency withimplementation of embodiments of some of the present techniques. Givenin Table 1 are representative energy-consumption data for an exemplarypolyethylene process technology utilizing a loop slurry reactor, a twinscrew extruder, and the typical associated equipment. Consumptionfigures in electricity are given in kilowatt-hours of electricityconsumed per metric ton of polyethylene produced. Consumption figuresfor steam are given in kilograms of steam consumed per metric ton ofpolyethylene produced. Lastly, fuel gas consumption is given inkilograms of fuel gas consumed per metric ton of polyethylene produced.

TABLE 1 Comparison of Energy Consumption Industry Avg. Improved¹Improved² Improved Avg. Electricity, kW-h/ton 458 325 350 338 Steam,kg/ton 312 144 148 146 Fuel Gas, kg/ton 7.6 2.8 6.5 4.7 ¹Exampledetermined August 2004; ²Example determined August 2005

An exemplary overall energy consumption number based on the combinationof electricity, steam, and fuel gas is 445 kW-h/ton based on the firstimproved example. The steam (e.g., 165 pounds per square inch absolute)consumption may be expressed as 144 kg/mt×900 Btu/lb×lb/0.454kg×0.000293 kw-hr/Btu=84 kw-h/mt. The fuel gas consumption (based on acombustible content of about 1,000 Btu per standard cubic feet) is 2.8kg/mt×1000 Btu/scf×359 scf/lb-mol×lb-mol/18 lbs×lb/0.454 kg×0.000293kw-hr/Btu=36 kw-h/mt. Therefore, the combined energy consumption is325+84+36=445 kw-h/mt. Similarly, for the second improved example, steamconsumption is 86 kw-h/mt and fuel gas consumption is 84 kw-h/mt.Therefore, for the second improved example, the combined energyconsumption based on electricity, steam, and fuel gas is 350+86+84=520kw-h/mt. Thus, for example 2, the electricity component is about ⅔ ofthe energy consumption, and steam and fuel gas are each about ⅙ of theenergy consumption. The total energy average of the two examples isabout 483 kw-h/mt.

Finally, it should be noted, as depicted in Table 2 below, that thepresent techniques provide for reduced usage of nitrogen and diluent(e.g., isobutane). Indeed, the present energy saving techniques offersignificant savings in the consumption of nitrogen and diluent. Forexample, with less pneumatic conveying loops for the fluff and pellets,less nitrogen is used and lost. Another example is with a majority ofthe recovered diluent bypassing the fractionation system via “direct”recycle to the polymerization reactor, less diluent is loss. Theexemplary consumption figures for nitrogen and diluent in Table 2 aregiven in normal cubic meters of nitrogen loss per metric ton ofpolyolefin produced, and in kilograms of isobutane diluent loss permetric ton of polyolefin produced, respectively.

TABLE II Nitrogen and Diluent Losses Industry Average Improved Nitrogen,Nm3/ton 76 26 Isobutane, kg/ton 13 1.7II. Polyolefin Production Process—An Overview

In the production of polyolefin, the polymerization reactor(s), whichpolymerize monomer into polyolefin, and the extruder(s), which convertthe polyolefin into polyolefin pellets, are typically continuousoperations. However, a variety of both continuous and batch systems maybe employed throughout the polyolefin process. An exemplary nominalcapacity for a typical polyolefin plant is about 600-800 million poundsof polyolefin produced per year. Exemplary hourly design rates areapproximately 85,000 to 90,000 pounds of polymerized polyolefin perhour, and 90,000 to 95,000 pounds of extruded polyolefin per hour.Again, in general, increases in production capacity may decrease theelectrical consumption in kilowatt-hours (kW-h) per pound of polyolefin.

Turning now to the drawings, and referring initially to FIG. 1, a blockdiagram depicts an exemplary manufacturing process 10 for producingpolyolefins, such as polyethylene homopolymer, polypropylenehomopolymer, and/or their copolymers. Various suppliers 12 may providereactor feedstocks 14 to the manufacturing system 10 via pipelines,trucks, cylinders, drums, and so forth. The suppliers 12 may compriseoff-site and/or on-site facilities, including olefin plants, refineries,catalyst plants, and the like. Examples of possible feedstocks 14include olefin monomers and comonomers (such as ethylene, propylene,butene, hexene, octene, and decene), diluents (such as propane,isobutane, n-hexane, and n-heptane), chain transfer agents (such ashydrogen), catalysts (such as Ziegler catalysts, Ziegler-Nattacatalysts, chromium catalysts, and metallocene catalysts), co-catalysts(such as triethylaluminum alkyl, triethylboron, and methyl aluminoxane),and other additives. In the case of ethylene monomer, exemplary ethylenefeedstock may be supplied via pipeline at approximately 800-1450 poundsper square inch gauge (psig) at 45-65° F. Exemplary hydrogen feedstockmay also be supplied via pipeline, but at approximately 900-1000 psig at90-110° F. Of course, a variety of supply conditions may exist forethylene, hydrogen, and other feedstocks 14.

A. Feed System

The suppliers 12 typically provide feedstocks 14 to a reactor feedsystem 16, where the feedstocks 14 may be stored, such as in monomerstorage and feed tanks, diluent vessels, catalyst tanks, co-catalystcylinders and tanks, and so forth. In the system 16, the feedstocks 14may be treated or processed prior to their introduction as feed 18 intothe polymerization reactors. For example, feedstocks 14, such asmonomer, comonomer, and diluent, may be sent through treatment beds(e.g., molecular sieve beds, aluminum packing, etc.) to remove catalystpoisons. Such catalyst poisons may include, for example, water, oxygen,carbon monoxide, carbon dioxide, and organic compounds containingsulfur, oxygen, or halogens. The olefin monomer and comonomers may beliquid, gaseous, or a supercritical fluid, depending on the type ofreactor being fed. Also, it should be noted that typically only arelatively small amount of fresh make-up diluent as feedstock 14 isutilized, with a majority of the diluent fed to the polymerizationreactor recovered from the reactor effluent. Moreover, improvedtechniques for regenerating the treatment beds, as explained below,reduce the amount of inert components (e.g., nitrogen) placed into theflare header and thus reduce the amount of fuel gas consumed by theflare.

The feed system 16 may prepare or condition other feedstocks 14, such ascatalysts, for addition to the polymerization reactors. For example, acatalyst may be activated and then mixed with diluent (e.g., isobutaneor hexane) or mineral oil in catalyst preparation tanks. In the catalystarea of the manufacturing process 10, an example of increased energyefficiency, as discussed below, is the use of a larger catalystactivator that consumes less fuel gas than the traditionally-sizedactivator. Furthermore, the larger activator may displace two or moresmaller activators.

Further, the feed system 16 typically provides for metering andcontrolling the addition rate of the feedstocks 14 into thepolymerization reactor to maintain the desired reactor stability and/orto achieve the desired polyolefin properties or production rate. In yetanother example of increased energy efficiency, also discussed below, animproved technique for metering monomer (e.g., ethylene) flow to thepolymerization reactor consumes less energy. Instead of the traditionalflow orifice meter, which measures differential pressure across anorifice plate and which generally requires steam (or steam condensate)preheating of the monomer for accurate measurement, a mass meter (e.g.,a Coriolis meter by MicroMotion, Inc. of Boulder, Colo.) may be used tomeasure ethylene flow. Mass meters typically do not require preheatingof the ethylene monomer and, thus, save energy as compared to thetraditional flow orifice meter.

Furthermore, in operation, the feed system 16 may also store, treat, andmeter recovered reactor effluent for recycle to the reactor. Indeed,operations in the feed system 16 generally receive both feedstock 14 andrecovered reactor effluent streams. In total, the feedstocks 14 andrecovered reactor effluent are processed in the feed system 16 and fedas feed streams 18 (e.g., streams of monomer, comonomer, diluent,catalysts, co-catalysts, hydrogen, additives, or combinations thereof)to the reactor system 20.

B. Reactor System

The reactor system 20 may comprise one or more reactor vessels, such asliquid-phase or gas-phase reactors. The reactor system 20 may alsocomprise a combination of liquid and gas-phase reactors. If multiplereactors comprise the reactor system 20, the reactors may be arranged inseries, in parallel, or in any other suitable combination orconfiguration. In the polymerization reactor vessels, one or more olefinmonomers are polymerized to form a product comprising polymerparticulates, typically called fluff or granules. The fluff may possessone or more melt, physical, rheological, and/or mechanical properties ofinterest, such as density, melt index (MI), melt flow rate (MFR),copolymer or comonomer content, modulus, and crystallinity. The reactionconditions, such as temperature, pressure, flow rate, mechanicalagitation, product takeoff, component concentrations, polymer productionrate, and so forth, may be selected to achieve the desired fluffproperties. Examples of increased energy efficiency via reducedelectrical consumption in the reactor system 20, as presented below, isprovided by improved techniques for cooling and discharging thepolymerization mixture in the reactor.

In addition to the one or more olefin monomers, a catalyst thatfacilitates polymerization of the monomer is typically added to thereactor. The catalyst may be a particle suspended in the fluid mediumwithin the reactor. In general, Ziegler catalysts, Ziegler-Nattacatalysts, metallocenes, and other well-known polyolefin catalysts, aswell as co-catalysts, may be used. An example of such a catalyst is achromium oxide catalyst containing hexavalent chromium on a silicasupport.

Further, diluent may be fed into the reactor, typically a liquid-phasereactor. The diluent may be an inert hydrocarbon that is liquid atreaction conditions, such as isobutane, propane, n-pentane, i-pentane,neopentane, n-hexane, cyclohexane, cyclopentane, methylcyclopentane,ethylcyclohexane, and the like. The purpose of the diluent is generallyto suspend the catalyst particles and polymer within the reactor. Somepolymerization processes may not employ a separate diluent, such as inthe case of selected polypropylene production where the propylenemonomer itself may act as the diluent.

A motive device may be present within the reactor in the reactor system20. For example, within a liquid-phase reactor, such as a loop slurryreactor, an impeller may create a turbulent mixing zone within the fluidmedium. The impeller may be driven by a motor to propel the fluid mediumas well as any catalyst, polyolefin fluff, or other solid particulatessuspended within the fluid medium, through the closed loop of thereactor. Similarly, within a gas-phase reactor, such as a fluidized bedreactor or plug flow reactor, one or more paddles or stirrers may beused to mix the solid particles within the reactor. An example of atechnique to save energy is the application of a motive force to thefluid medium in a loop slurry reactor via a single larger pump in lieuof two smaller pumps (in series), thus saving electrical costs.

C. Diluent/Monomer Recovery, Treatment, and Recycle

The discharge 22 of the reactors within system 20 may include thepolymer fluff as well as non-polymer components, such as diluent,unreacted monomer/comonomer, and residual catalyst. The discharge 22 maybe subsequently processed, such as by a diluent/monomer recovery system24, to separate non-polymer components 26 (e.g., diluent and unreactedmonomer) from the polymer fluff 28. In the diluent/monomer recoverysystem 24, increases in energy efficiency have been accomplished bydecreasing electrical consumption via elimination of processing steps,such as with the elimination of a low-pressure recovery flash of thediluent/monomer and the associated recycle compression.

With or without the low pressure flash, the untreated recoverednon-polymer components 26 may be further processed, such as by afractionation system 30, to remove undesirable heavy and lightcomponents. Fractionated product streams 32 may then be returned to thereactor system 20 via the feed system 16. On the other hand, thenon-polymer components 26 may recycle more directly to the feed system16 (as indicated by reference numeral 34), bypassing the fractionationsystem 30, and thus avoiding the energy consumption of the fractionationsystem 30. Indeed, in certain embodiments, up to 80-95% of the diluentdischarged from the reactor bypasses the fractionation system in routeto the polymerization reactor. As a result, the size of thefractionation columns and associated steam consumption in the downstreamfractionation system 30 may be reduced by as much as 70-90%.

As for the fluff 28, it may be further processed within the recoverysystem 24 and in the extrusion/loadout system 36, to prepare it forshipment, typically as pellets 38, to customers 40. Although notillustrated, polymer granules intermediate in the recovery system 24 andtypically containing active residual catalyst may be returned to thereactor system 20 for further polymerization, such as in a differenttype of reactor or under different reaction conditions. Thepolymerization and diluent recovery portions of the polyolefinmanufacturing process 10 may be called the “wet” end 42 or “reaction”side of the process 10, and the extrusion/loadout 36 portion of thepolyolefin process 10 may be called the “dry” end 44 or “finishing” sideof the polyolefin process 10.

Exemplary techniques directed toward increasing energy efficiency mayinclude utilization of process pressure, instead of anelectrical-mechanical motive force (e.g., blower or compressor), totransport or convey the polymer fluff 28 from the recovery system 24 tothe extrusion/loadout system 36. Improved techniques may also includeoperation of the reactor system 20 to provide for more direct couplingin operation of the reactor system 20 and recovery system 24 to theextruder/loadout system 36. Such direct or “close” operative couplingmay reduce the need for process residence time of the fluff 28. Thus,the number of intermediate fluff storage vessels (e.g., silos) andassociated blower/compressor systems and electrical consumption may bereduced.

D. Extrusion/Loadout System

In the extrusion/loadout systems 36, the fluff 28 is typically extrudedto produce polymer pellets 38 with the desired mechanical, physical, andmelt characteristics. Extruder feed may comprise additives, such as UVinhibitors and peroxides, which are added to the fluff products 28 toimpart desired characteristics to the extruded polymer pellets 32. Anextruder/pelletizer receives the extruder feed, comprising one or morefluff products 28 and whatever additives have been added. Theextruder/pelletizer heats and melts the extruder feed which then may beextruded (e.g., via a twin screw extruder) through a pelletizer dieunder pressure to form polyolefin pellets. Such pellets are typicallycooled in a water system disposed at or near the discharge of thepelletizer. An exemplary energy-saving technique includes the use of apellet water pump (e.g., having a 15-50 horsepower motor) to transportthe extruder pellets in the pellet water to the loadout area. This iscontrast to traditional approach of employing a conventional conveyingloop which typically uses a pellet blower operating at about 250-500horsepower.

In general, the polyolefin pellets may then be transported to a productload-out area where the pellets may be stored, blended with otherpellets, and/or loaded into railcars, trucks, bags, and so forth, fordistribution to customers 40. In the case of polyethylene, pellets 38shipped to customers 40 may include low density polyethylene (LDPE),linear low density polyethylene (LLDPE), medium density polyethylene(MDPE), high density polyethylene (HDPE), and enhanced polyethylene. Thevarious types and grades of polyethylene pellets 38 may be marketed, forexample, under the brand names Marlex® polyethylene or MarFlex™polyethylene of Chevron-Phillips Chemical Company, LP, of The Woodlands,Tex., USA.

E. Customers, Applications, and End-Uses

Polyolefin (e.g., polyethylene) pellets 38 may be used in themanufacturing of a variety of products, components, household items andother items, including adhesives (e.g., hot-melt adhesive applications),electrical wire and cable, agricultural films, shrink film, stretchfilm, food packaging films, flexible food packaging, milk containers,frozen-food packaging, trash and can liners, grocery bags, heavy-dutysacks, plastic bottles, safety equipment, coatings, toys and an array ofcontainers and plastic products. Further, it should be emphasized thatpolyolefins other than polyethylene, such as polypropylene, may formsuch components and products via the processes discussed below.

Ultimately, the products and components formed from polyolefin (e.g.,polyethylene) pellets 38 may be further processed and assembled fordistribution and sale to the consumer. For example, a polyethylene milkbottle may be filled with milk for distribution to the consumer, or thefuel tank may be assembled into an automobile for distribution and saleto the consumer.

To form end-products or components from the pellets 38, the pellets aregenerally subjected to further processing, such as blow molding,injection molding, rotational molding, blown film, cast film, extrusion(e.g., sheet extrusion, pipe and corrugated extrusion,coating/lamination extrusion, etc.), and so on. Blow molding is aprocess used for producing hollow plastic parts. The process typicallyemploys blow molding equipment, such as reciprocating screw machines,accumulator head machines, and so on. The blow molding process may betailored to meet the customer's needs, and to manufacture productsranging from the plastic milk bottles to the automotive fuel tanksmentioned above. Similarly, in injection molding, products andcomponents may be molded for a wide range of applications, includingcontainers, food and chemical packaging, toys, automotive, crates, capsand closures, to name a few.

Extrusion processes may also be used. Polyethylene pipe, for example,may be extruded from polyethylene pellet resins and used in anassortment of applications due to its chemical resistance, relative easeof installation, durability and cost advantages, and the like. Indeed,plastic polyethylene piping has achieved significant use for watermains, gas distribution, storm and sanitary sewers, interior plumbing,electrical conduits, power and communications ducts, chilled waterpiping, well casing, to name a few applications. In particular,high-density polyethylene (HDPE), which generally constitutes thelargest volume of the polyolefin group of plastics used for pipe, istough, abrasion-resistant and flexible (even at subfreezingtemperatures). Furthermore, HDPE pipe may be used in small diametertubing and in pipe up to more than 8 feet in diameter. In general,polyethylene pellets (resins) may be supplied for the pressure pipingmarkets, such as in natural gas distribution, and for the non-pressurepiping markets, such as for conduit and corrugated piping.

Rotational molding is a high-temperature, low-pressure process used toform hollow parts through the application of heat to biaxially-rotatedmolds. Polyethylene pellet resins generally applicable in this processare those resins that flow together in the absence of pressure whenmelted to form a bubble-free part. Pellets 38, such as certain Marlex®HDPE and MDPE resins, offer such flow characteristics, as well as a wideprocessing window. Furthermore, these polyethylene resins suitable forrotational molding may exhibit desirable low-temperature impactstrength, good load-bearing properties, and good ultraviolet (UV)stability. Accordingly, applications for rotationally-molded Marlex®resins include agricultural tanks, industrial chemical tanks, potablewater storage tanks, industrial waste containers, recreationalequipment, marine products, plus many more.

Sheet extrusion is a technique for making flat plastic sheets from avariety of pellet 38 resins. The relatively thin gauge sheets aregenerally thermoformed into packaging applications such as drink cups,deli containers, produce trays, baby wipe containers and margarine tubs.Other markets for sheet extrusion of polyolefin include those thatutilize relatively thicker sheets for industrial and recreationalapplications, such as truck bed liners, pallets, automotive dunnage,playground equipment, and boats. A third use for extruded sheet, forexample, is in geomembranes, where flat-sheet polyethylene material iswelded into large containment systems for mining applications andmunicipal waste disposal.

The blown film process is a relatively diverse conversion system usedfor polyethylene. The American Society for Testing and Materials (ASTM)defines films as less than 0.254 millimeter (10 mils) in thickness.However, the blown film process can produce materials as thick as 0.5millimeter (20 mils), and higher. Furthermore, blow molding inconjunction with monolayer and/or multilayer coextrusion technologieslay the groundwork for several applications. Advantageous properties ofthe blow molding products may include clarity, strength, tearability,optical properties, and toughness, to name a few. Applications mayinclude food and retail packaging, industrial packaging, andnon-packaging applications, such as agricultural films, hygiene film,and so forth.

The cast film process may differ from the blown film process through thefast quench and virtual unidirectional orientation capabilities. Thesecharacteristics allow a cast film line, for example, to operate athigher production rates while producing beneficial optics. Applicationsin food and retail packaging take advantage of these strengths. Finally,polyolefin pellets may also be supplied for the extrusion coating andlamination industry.

III. Polymerization Reactor Feed System

A. Monomer Feed

Referring to FIG. 2, a process flow diagram of an exemplary reactor feedsystem 16 (of FIG. 1) is depicted. In this embodiment, monomer 50 (e.g.,ethylene) is fed through monomer treaters 52 to the liquid phase reactor(e.g., loop slurry reactor) in the reactor system 20. Furthermore, amass flow meter 53, instead of an orifice plate meter, may be used tomeasure the flow rate of ethylene to the reactor.

Indeed, the flow rate of ethylene monomer 50 to the reactor generally istypically measured (and controlled) to facilitate desired operatingconditions (e.g., slurry density, comonomer/monomer ratio, productionrate, etc.) in the reactor and to provide the desired properties of thepolyethylene formed in the reactor. The exemplary mass flow meter 53used to measure the ethylene monomer flow may be a Coriolis meter, forexample. A Coriolis meter typically does not utilize preheating of theethylene for accurate measurement of the ethylene flow rate. Incontrast, as will be appreciated by those of ordinary skill in the art,an orifice plate meter generally utilizes preheating of the ethylenebecause the typical feedstock conditions of the ethylene may operateclose to the critical point of ethylene (i.e., critical pressure andcritical temperature). For example, with the orifice plate, the flowindication is typically inaccurate if the ethylene is operating close toits critical point because of the rapid change of fluid density near thecritical point. In contrast, the Coriolis mass flow meter is tolerant ofdensity changes or even phase changes because of its working principalwhich measures mass instead of pressure drop like the orifice plate(which is influenced by density or phase of the fluid). In addition tosaving steam or steam condensate in not having to preheat the ethylenemonomer feed, the avoidance of heating the ethylene results in coolerfeed to the polyethylene reactor, and thus less heat needs to be removedfrom the reactor during polymerization, advancing further energysavings.

Coriolis flow meters are readily available and provide for low pressuredrop, ease of installation, cleanability, and drainability. Accuracy forCoriolis meters is typically in the range of 0.05% to 0.4% with turndownratios up to 200. Major suppliers of Coriolis mass meters include, forexample: MicroMotion Company of Boulder, Colo.; Endress Hauser Companyof Greenwood, Ind.; FMC/Direct Measurement Company, Longmont, Colo.; andLiquid Controls, Inc., of Lake Bluff, Ill.

B. Other Feed Streams

Recycle diluent 54 (e.g., isobutane) with a relatively small amount ofentrained monomer may be returned from the diluent/monomer recoverysystem 24 (e.g., corresponding to stream 34 of FIG. 1) and sent to thepolymerization reactor. In the example of “direct” recycle to thereactor, the recycled diluent 54 may be cooled and passed through aheavies knockout pot 56, where heavy components are removed out of abottom discharge and sent via a centrifugal pump 58, for example, asfeed 59 to the fractionation system 30. The overhead 62 of the knockoutpot 56 may be further cooled in a heat exchanger 66 and collected in arecycle diluent surge tank 68 for feed to the reactor. Downstream, acentrifugal pump 70 may deliver the diluent 72 through recycle diluenttreaters 74 to the loop slurry reactor. It should be noted that arelatively small amount of fresh diluent (not illustrated) may be addedin the fractionation system 30, for example, to make-up for diluentlosses in the manufacturing process 10. Furthermore, comonomer 76 (e.g.,1-hexene) may be added to the suction of pump 70 or at other points inthe recycle diluent circuit for addition to the reactor. The monomertreaters 52 and recycle diluent treaters 58 may include molecular sieveor aluminum packing, for example, configured to remove catalyst poisonsfrom the monomer, recycle diluent, comonomer feeds, and other feeds.

Other feed components may be added to the loop slurry reactor. Forexample, hydrogen 60 may be added to control the molecular weight of thepolyolefin formed in the reactor. Furthermore, other additives, such asantistatic materials, may be injected into the reactor, as indicated byreference numeral 78. The various component streams may combine into asingle feed stream 80 for feed to the loop slurry reactor. Further, asdiscussed below, diluent 82 that is substantially olefin-free may berecycled from the fractionation system 30 through treaters 84 for use inthe preparation of the catalyst fed to the reactor. Indeed, diluent 82may act as a carrier of the catalyst stream 88 discharged from thecatalyst preparation system 86 in route to the loop slurry reactor.

C. Feed Treaters

Traditionally, up to 12-15 treaters have been employed to process thevarious feeds. For example, additional treaters have been utilized toremove poisons from the comonomer, fresh isobutane, and hydrogen.However, the total number of treaters may be reduced to six treaters, asillustrated in FIG. 2. This reduction in the number of treaters may beimplemented by combining treatment of the fresh diluent with the recyclediluent, and using reactor grade hydrogen that is relatively free ofpoisons, and so on. Further, as illustrated, the monomer and diluentstreams may each be configured with one treater and one spare treater,for a total of four treaters. Note that the illustrated six treaters maybe further reduced to five treaters by sharing a spare treater betweenthe monomer and diluent streams 50 and 72. Other configurations mayemploy even less treaters. In general, a reduction in the number oftreaters reduces capital costs and saves energy by lowering electricalconsumption due to more efficient scalability in regeneration (whichtypically uses electrical heat) of the treaters.

Moreover, an improved technique for regenerating the various treaters52, 74, and 84, as well as other treaters, involves lessening the inertcomponent load on the flare system to reduce fuel gas consumption. Thismay be accomplished by discharging regeneration nitrogen from thetreaters to the atmosphere instead of to the flare header. In general,an inert component, such as nitrogen, is typically used to regeneratethe various treaters. This nitrogen has traditionally been placed intothe flare header which increases the inert component load on the flare.As will be appreciated by those of ordinary skill in the art, thecombustible content, i.e., the British Thermal Units (BTU's), of thefeed to the flare is generally maintained at an acceptable minimum levelto avoid adversely affecting operation of the flare. Therefore, toaccount for an increased concentration of inert components in the flareheader (e.g., due to the injection of regeneration nitrogen into theheader), fuel gas (e.g., natural gas) is added to the flare header. Thepresent technique provides for discharging less of the nitrogen used forregeneration into the header, and thus reduces the consumption of fuelgas.

Referring to FIG. 3, a process flow diagram of a treater regenerationsystem 100 is depicted. Nitrogen 102 is passed through a cross-exchanger104, where it is preheated by the hot nitrogen discharging from thetreaters (e.g., 52, 74 and 84). The preheated nitrogen 106 then enters aregeneration heater 108, where an electrical element 110 heats thenitrogen via a controller 112. In general, the nitrogen 102 may besupplied from a main nitrogen header, for example, at nominally 150pounds per square inch gauge (psig) and at ambient temperature. Thenitrogen may be heated in the electric regeneration heater 110 up toabout 600° F. for regeneration. The heated nitrogen 114 enters thetreaters to regenerate the molecular sieve or desiccant. The hotnitrogen 116 containing the removed catalyst poisons from the treatersenters the cross exchanger 104, exiting as spent stream 118 which may bedischarged to the flare 120.

On the other hand, clean nitrogen exiting during the subsequentcool-down of the treaters may discharge to the atmosphere 122. Ingeneral, the regeneration of the treaters occurs while the hot nitrogenis circulated through the treaters. Upon completion of regeneration,cool nitrogen is typically circulated through the treaters to cool thetreater in preparation for normal operation.

Referring to FIG. 4, a treater regeneration method 130 is depicted. Uponsaturation of the molecular sieve packing with catalysts poison andother components, the various treaters may be regenerated. Initially,the process may take the saturated treater out of service and switchoperation to the spare treater, as referenced in block 132. Hot nitrogenmay be passed through the spent treater, and the hot nitrogen dischargedto the flare (block 134). The amount of time hot nitrogen is passedthrough the treater may be influenced by a variety of factors, such astreater bed temperatures. In general, the hot regeneration isdiscontinued, and the treater prepared for cool down, when the bedoutlet temperature reaches 450 to 500° F., and has been held at thattemperature for at least two hours. Such procedures, however, may varyfrom plant to plant. Moreover, total regeneration time may depend alsoon the treater/bed size, regeneration heater capacity, nitrogen flow,weight, and the quality and thickness of the insulation surrounding thetreaters, and so forth. In the example of an ethylene monomer treater 52and/or a recycle diluent treater 74, the hot nitrogen may be passedthrough the heater for roughly about 18-30 hours.

Upon dissipation of the catalyst poisons from the treater bed via thehot nitrogen regeneration, the treater is prepared for cool-down. Coolnitrogen is passed through the treater to cool the treater prior toplacing the treater back into operation (block 136). In the example ofan ethylene monomer treater 54 and/or a recycled diluent treater 72, thecool-down time with nitrogen is about 8-16 hours. However, again, thetiming of the cool down cycle may vary considerably from plant to plant.Traditionally, this clean nitrogen exiting the treater has beendischarged to the flare header, as with the hot nitrogen. The presenttechnique, however, provides for discharging the clean nitrogen to theatmosphere instead of to the flare header, and thus reduces the inertload on the flare. Thus, as discussed, fuel gas consumption isdecreased.

D. Polymerization Catalyst

Referring to FIG. 5, a process flow diagram of the catalyst preparationarea 86 is depicted. A catalyst mix tank 140 receives catalyst 142, forexample, from a portable container. Olefin-free monomer 82 mixes withthe catalyst in the catalyst mix tank 140. An agitator 144 having amotor and drive 146 and blade 148 may facilitate mixing of the diluent82 and the catalyst 142 in the mix tank 140. The process catalyst 150discharges from the mix tank 140 and may enter, for example, a catalystrun tank 152 for metering to the loop slurry reactor. The run tank 152may also have an agitator 154 having a motor/drive 156 and agitatorblade 158 to maintain the catalyst mixed with the diluent. The catalystmay be metered, for example, by a positive displacement pump 160 to theloop slurry reactor as feed stream 88. Additionally, additives, such asco-catalysts (e.g., triethylaluminum) 162, may be added to the catalyst88 fed to the reactor. Finally, it should be noted that prior to mixingand metering the catalyst, the catalyst may be activated. For example,in the case of a chromium oxide catalyst, a catalyst activator mayconvert the chromium Cr3+ to Cr6+ for injection into the polymerizationreactor. While in the reactor and in contact with the ethylene monomer,for example, the chromium Cr6+ may reduce to Cr2+.

Referring to FIG. 6, a process flow diagram of a catalyst activatorsystem 170 is depicted. The activated catalyst product of system 170 isfed to the catalyst mix tank 140 (catalyst 142) of FIG. 5. In FIG. 6,the catalyst activator includes an internal vessel 172 containing thecatalyst, and an external furnace 174. Catalyst from the supplier may beheld in a holding vessel 176 and fed via to the internal vessel via anon/off valve 178, for example. Fuel 180 may be added via a sparger orpilot 182, for example, into the furnace 180, and the fuel 180 may becombined with air 184 injected into the furnace via an air filter 186and air blower 188. Combustion may take place inside the furnace in theregion 190, for example. The region 192 surrounding the internal vessel172 may experience operating temperatures in an exemplary range of 800to 1700° F. The heated fluid from this region 192 may discharge to theatmosphere 194, as depicted by arrow 196.

In addition to high heat, oxygen may be supplied to activate thecatalyst. Air 198 may be injected into the bottom of the internal vessel172 to provide the presence of oxygen inside the vessel, with heatprovided by the surrounding furnace 174. The air entering the vessel 172may exit at the top via an internal air filter 200, for example, andthen discharge to the atmosphere, as indicated by reference numeral 202.The activated catalyst may discharge from vessel 172 into a catalysttote bin 206, or other container. Furthermore, nitrogen 208 mayfacilitate discharge of the activated catalyst into the tote bin 206,and also provide an inert atmosphere in the tote bin 206.

In general, catalyst activation processes include passing dry airthrough a catalyst bed at a constant rate, while applying heat, untilthe catalyst reaches the desired temperature, at which point thecatalyst is held at the activation temperature for the proper length oftime. A technique to improve the energy efficient of catalyst activationis to increase the diameter of the inner vessel 172. Conventionally, thenominal inner diameter of the vessel 172 has been about 42 inches orless. The present technique provides for increasing the nominal innerdiameter (ID) of vessel 172 up to about 48-72 inches and greater. Forthe example of a 60 inch nominal ID, the catalyst throughput rate ofcatalyst may increased by 50-100%, while the fuel gas 180 consumed inthe furnace remains essentially constant or slightly increases (i.e.,less than 10% increase). Thus, the catalyst activation capacity isincreased by 50-100%, while the furnace duty value remains at about thetraditional 5 million BTU's per hour. The technique gives significantimprovement in fuel gas efficiency in the manufacturing process 10.Indeed, in certain embodiments, the catalyst activation system 170 isgenerally the largest consumer of fuel gas in the polyolefinmanufacturing process 10.

IV. Polymerization Reactor System

Referring to FIG. 7, a process flow diagram of an exemplarypolymerization reactor system 20 (of FIG. 1) and diluent/monomerrecovery system 24 (also of FIG. 1) are depicted. As discussed above,the reactor system 20 may comprise one or more polymerization reactors,which may in turn be of the same or different types. Furthermore, inmultiple reactor systems, the reactors may be arranged serially or inparallel. Whatever the reactor types comprising the reactor system 20, apolyolefin particulate product, generically referred to as “fluff”herein, is produced. To facilitate explanation, the following examplesare limited in scope to specific reactor types believed to be familiarto those skilled in the art and to single reactors or simplecombinations. To one of ordinary skill in the art using this disclosure,however, the present techniques are simply and easily applicable to morecomplex reactor arrangements, such as those involving additionalreactors, different reactor types, and/or alternative ordering of thereactors or reactor types. Such arrangements are considered to be wellwithin the scope of the present invention.

One reactor type comprises reactors within which polymerization occurswithin a liquid phase. Examples of such liquid phase reactors includeautoclaves, boiling liquid-pool reactors, loop slurry reactors (verticalor horizontal), and so forth. For simplicity, a loop slurry reactor 210which produces polyolefin, such as polyethylene, polypropylene, andtheir copolymers, will be discussed in the context of the presenttechniques though it is to be understood that the present techniques aresimilarly applicable to other types of liquid phase reactors.

The loop slurry reactor 210 is generally composed of segments of pipeconnected by smooth bends or elbows. An exemplary reactor 210configuration includes eight jacketed vertical pipe legs, approximately24 inches in diameter and approximately 200 feet in length, connected bypipe elbows at the top and bottom of the legs. As discussed below,reactor jackets 212 are normally provided to remove heat from theexothermic polymerization via circulation of a cooling medium, such astreated water, through the reactor jackets 212.

The reactor 210 may be used to carry out polyolefin polymerization underslurry conditions in which insoluble particles of polyolefin are formedin a fluid medium and are suspended as slurry until removed. A motivedevice, such as pump 214, circulates the fluid slurry in the reactor210. An example of a pump 214 is an in-line axial flow pump with thepump impeller disposed within the interior of the reactor 210 to createa turbulent mixing zone within the fluid medium. The impeller may alsoassist in propelling the fluid medium through the closed loop of thereactor at sufficient speed to keep solid particulates, such as thecatalyst or polyolefin product, suspended within the fluid medium. Theimpeller may be driven by a motor 216 or other motive force.

The fluid medium within the reactor 210 may include olefin monomers andcomonomers, diluent, co-catalysts (e.g., alkyls, triethylboron, methylaluminoxane, etc.), molecular weight control agents (e.g., hydrogen),and any other desired co-reactants or additives. Such olefin monomersand comonomers are generally 1-olefins having up to 10 carbon atoms permolecule and typically no branching nearer the double bond than the4-position. Examples of monomers and comonomers include ethylene,propylene, butene, 1-pentene, 1-hexene, 1-octene, and 1-decene. Again,typical diluents are hydrocarbons which are inert and liquid underreaction conditions, and include, for example, isobutane, propane,n-pentane, i-pentane, neopentane, n-hexane, cyclohexane, cyclopentane,methylcyclopentane, ethylcyclohexane, and the like. These components areadded to the reactor interior via inlets or conduits at specifiedlocations, such as depicted at feed stream 80, which generallycorresponds to one of the feed streams 18 of FIG. 1. Likewise, acatalyst, such as those previously discussed, may be added to thereactor 210 via a conduit at a suitable location, such as depicted atfeed stream 88, which may include a diluent carrier and which alsogenerally corresponds to one of the feed streams 18 of FIG. 1. In total,the added components generally compose a fluid medium within the reactor210 within which the catalyst is a suspended particle.

The reaction conditions, such as temperature, pressure, and reactantconcentrations, are regulated to facilitate the desired properties andproduction rate of the polyolefin in the reactor, to control stabilityof the reactor, and the like. Temperature is typically maintained belowthat level at which the polymer product would go into solution. Asindicated, due to the exothermic nature of the polymerization reaction,a cooling fluid may be circulated through jackets 212 around portions ofthe loop slurry reactor 210 to remove excess heat, thereby maintainingthe temperature within the desired range, generally between 150° F. to250° F. (65° C. to 121° C.). Likewise, pressure may be regulated withina desired pressure range, generally 100 to 800 psig, with a range of450-700 psig being typical. To reduce electrical consumption in thereactor system 20, a coolant pump that circulates the treated waterthrough the jackets 212 may be reduced in size by lowering the flow rateof water (e.g., by half) and permitting a greater temperature increaseΔT of water (e.g., ΔT in the range of 15 to 45° F., instead of a typical10° F.). Thus, in one embodiment, the horsepower of the coolant pumpmotor (see FIGS. 8 and 9) may be reduced by 30-70%.

As the polymerization reaction proceeds within the reactor 210, themonomer (e.g., ethylene) and comonomers (e.g., 1-hexene) polymerize toform polyolefin (e.g., polyethylene) polymers that are substantiallyinsoluble in the fluid medium at the reaction temperature, therebyforming a slurry of solid particulates within the medium. These solidpolyolefin particulates may be removed from the reactor 210 via asettling leg or other means, such as a continuous take-off, as depicteddischarge stream 22. In downstream processing, the polyethylenedischarged from the reactor may be extracted from the slurry andpurified.

V. Diluent/Monomer Recovery System

A. Flash Chamber

The discharge 22 from the reactor 210 may flow through an in-line flashheater 222 and into a flash chamber 224. The in-line flash heater 222may be a surrounding conduit that uses steam or steam condensate, forexample, as a heating medium to provide indirect heating to thedischarge 22. Thus, the loop slurry reactor 210 effluent (discharge 22)is heated prior to its introduction into the flash chamber 224. Also,before the discharge 22 enters the flash chamber 224, water or othercatalysts poisons may be injected into the discharge 22 to deactivateany residual catalysts in the discharge 22 stream. Because theseinjected components are catalysts poisons by definition, they aretypically completely removed, or at least substantially removed, fromany recovered material (e.g., monomer or diluent) recycled to thereactor 210.

In the flash chamber 224, most of the non-solid components of thereactor discharge 22 are withdrawn overhead as vapor in the flash gas226. Note, it is this recycled flash gas 226 that may bypass thefractionation system in route to the reactor 210 (i.e., via the feedsystem 16). In polyethylene production, this vapor is typicallyprimarily diluent, such as isobutane or other diluents previouslymentioned. It may also contain most of the unreacted monomer (e.g.,ethylene) and other light components, as well as unreacted comonomer(e.g., 1-hexene, butene, 1-pentene, 1-octene, and 1-decene) and otherheavy components (e.g., hexane and oligomers). In general lightcomponents or “lights” may be defined at those light components withlower boiling points than the diluent employed. In contrast heavycomponents or “heavies” may be defined as those components having higherboiling points than the diluent. An exemplary approximate composition ofthe flash gas 226 is 94 wt. % isobutane, 5 wt. % ethylene, and 1 wt. %other components. A level or volume of fluff may be maintained in theflash chamber 224 to give additional residence time of the fluff in thechamber 224 to facilitate separation of liquid and vapor entrained inthe porous fluff particles.

The flash gas 226 may be processed in equipment such as cyclones, bagfilters, etc., where entrained fluff solids are removed and returned tothe flash chamber 224 or to downstream equipment, such as the purgecolumn discussed below. The flash gas 226 may also travel through adeoxygenation bed, for example. Furthermore, the flash gas 226 may becooled or condensed in a heat exchanger (e.g., shell-and-tubeconstruction) prior to its recycle to the feed system 16 orfractionation system 30. To reduce steam consumption in thefractionation system 30, the flash gas 226 may bypass the fractionationsystem 30 and return more directly to the reactor 210 via the feedsystem 16.

As for the solids (polymer) in the flash chamber 224, they are withdrawnwith a small amount of entrained diluent (and monomer) and sent to apurge column 228 via solids discharge 230. As will be appreciated bythose of ordinary skill in the art, the solids discharge 230 conduit mayinclude valve configurations that allow polymer to flow downward throughthe conduit while reducing the potential for vapor to flow between thepurge column 228 and the flash chamber 224. For example, one or morerotary or cycling valves may be disposed on the solids discharge 230conduit. Furthermore a relatively small fluff chamber may also bedisposed on the conduit. Traditionally, the fluff solids from the flashchamber has discharged into a lower pressure flash chamber, with thelower pressure flash gas requiring compression for recycle tofractionation system 30 and reactor. However, the present techniquesprovide for elimination of a low pressure flash and the associatedcompression (a significant consumer of electricity), and discharge ofthe fluff solids from the flash chamber 224 to the purge column 228.Such a discharge to the purge column may include appropriate valveconfigurations, a surge chamber, or simply a conduit, and so on. Notethat certain embodiments provide for a continuous fluff discharge fromthe flash chamber, which eliminates one or more relatively large cyclingvalves and the associated energy consumption.

B. Purge Column

The primary solids feed to the purge column 228 is typically the solidsdischarge 230 (polyolefin fluff) that exits the flash chamber 224. Apurpose of the purge column 228 is to remove residual hydrocarbon fromthe entering solids streams and to provide substantially-clean polymerfluff 232. The fluff 232 may be transported or conveyed to theextrusion/loadout system 36 for conversion to pellets 38, and fordistribution and sale as polyolefin pellet resin to customers 40. Ingeneral, the treated polymer particles discharged from purge column 228as polymer fluff 232 may be processed in a conventional finishingoperation, such as a screw extruder, in the extrusion/load out system 36(FIG. 1).

In the exemplary purge column system illustrated, nitrogen is circulatedthrough purge column 228 to remove residual hydrocarbons via overheaddischarge 234. This discharge 234 may be sent through a separation unit236, such as a membrane recovery unit, pressure swing adsorption unit,refrigeration unit, and so forth, to recover nitrogen via nitrogenstream 238, and to discharge a separated hydrocarbon stream 240 as feedto the fractionation system 30. In the art, the separation unit 236 maybe known as an Isobutane Nitrogen Recovery Unit (INRU). Moreover, freshnitrogen 242 may be added to the nitrogen circuit to account fornitrogen losses in the purge column 228 system. Finally, it should benoted that the hydrocarbon stream 240 may beneficially provide feed tothe fractionation system 30 (see FIG. 13). For example, the hydrocarbonstream 240 discharging from the separation unit 236 makes availablehydrocarbon feed that may be processed to give the olefin-free diluentused in catalyst preparation.

C. Alternate Configurations

As will be appreciated by those of ordinary skill in the art, a varietyof configurations may be employed in the diluent/monomer recovery system24. For example, the solids discharge 230 from the flash chamber 224 maybe sent to another reactor (e.g., a gas phase reactor) instead of to thepurge column 228 or to a low-pressure flash chamber. If discharged toanother reactor, catalyst poison may not be injected upstream in thedischarge 22, and, thus, residual active catalysts remain for furtherpolymerization.

In another configuration, the purge column 228 may be eliminated fromthe recovery system 20 and combined with the downstream extruder feedtank. The separation unit 236 associated with the purge column 228 maybe relocated to accommodate the extruder feed tank if desired. Thus, thehigh process pressure in the flash chamber 224 may be utilized to conveythe fluff particles in solids discharge 230 to the extrusion/loadoutsystem 36, eliminating a blower system (and associated electricalconsumption) traditionally used to convey the fluff 232 to theextrusion/loadout system. Furthermore, heat in the fluff particles maybe retained as the particles are not subjected to the typical coolingeffect of nitrogen in a conventional, blower conveying loop. Thus, lessheating of the fluff particles may be used in the downstream extruderfeed system. Finally, the process pressure in the flash chamber 224 maybe used to transport the fluff particles in a dense phase conveyingarrangement, thus lowering the velocity of the flowing particles andreducing transport damage to the particles.

VI. Reactor Cooling

An improved technique for reactor cooling provides for a smaller coolantpump and associated motor, and thus a reduction in electricalconsumption. To accomplish installation and operation of a smallercoolant pump (i.e., cooling water pump), the temperature of the coolingwater through the reactor jackets is allowed to increase more than theconventional design specification of 10° F. Thus, the cooling water flow(and the size of the coolant pump) may be reduced.

A. Loop Slurry Reactor

FIG. 8 depicts an exemplary polymerization reactor 210 of FIG. 7 andshows a counter-current flow scheme of cooling medium through thereactor jackets 212A-H. Again, the loop reactor 210 is generallycomposed of segments of pipe connected by smooth bends or elbows. Amotive device, such as pump 214, circulates the fluid slurry in thereactor 210. An example of a pump 214 is an in-line axial flow pump withthe pump impeller disposed within the interior of the reactor 210. Acoolant system 250 removes heat from the loop reactor 210 via reactorjackets 212A-H. The coolant system 250 provides a coolant supply 252(e.g., treated water) and processes a coolant return 254.

As the polymerization reaction proceeds within the reactor 210, thereaction conditions may be controlled to facilitate the desired degreeof polymerization and the desired reaction speed while keeping thetemperature below that at which the polymer product would go intosolution. As mentioned, due to the exothermic nature of thepolymerization reaction, cooling jackets 212A-H may be provided aroundportions of the closed loop system through which a cooling fluid iscirculated as needed to remove excess heat (heat of reaction), therebymaintaining the temperature within the desired range, generally between150° F. to 250° F. (65° C. to 121° C.).

In general, reactor temperature varies linearly with changes in thereactor system operating conditions. An accepted assumption in the artis that heat generated in the reactor by the exothermic polymerizationis linear with the polyolefin production rate (i.e., pounds per hour ofpolyolefin polymerized). Thus, reactor temperature, which is anindication of the energy or heat in the reactor, varies linearly withproduction rate. As appreciated by those of ordinary skill in the art,typical reactor temperature control may involve aproportional-integral-derivative (PID) algorithm.

B. Reactor Coolant System

Referring to FIG. 9, a process flow diagram of a coolant system 250 forthe loop slurry reactor 210 of FIG. 8 is depicted. Coolant system 250provides coolant supply 252 to reactor jackets 212A-H. Coolant system250 receives coolant return 254 from reactor jackets 212A-H. A varietyof coolants may be used to remove or add heat to the reactor system. Inthis illustrative embodiment, steam condensate (demineralized water) isused as the coolant. The coolant return 254 “carries” the heat removedfrom the reactor. The coolant system 250 transfers this heat to autility cooling medium, such as to cooling tower water or sea water. Thecoolant system delivers “cooled” coolant supply 252 to the reactorjackets. Typical coolant supply 252 temperatures range from 105° F. to150° F. and typical coolant return 254 temperatures range from 130° F.to 190° F.

Coolant flow through the coolant system 250 and through the reactorjackets 212A-H may be circulated, for example, by a centrifugal pump, asillustrated by coolant pump 256. An exemplary design basis of a coolantpump 256 is approximately 50 to 60 pounds per square inch (psi)delivered head at 3 to 12 million pounds per hour of coolant. An exampleconfiguration of the reactor jackets 212A-H (FIG. 8) is twocounter-current double-pipe exchangers operated in parallel, with theinner pipe (the reactor) having an approximate 22 inch internaldiameter, and the outer pipe (the jacket) having an approximate 28 inchinternal diameter. In this example, the total heat transfer area of thereactor jackets 212A-His about 5,000 square feet.

The coolant circulation may be a closed loop, hydraulically full system.A surge drum may be employed in the coolant circuit (i.e., at or nearthe suction of pump 256) to maintain the circuit liquid full and toreduce swings in pressure of the coolant system by compensating forhydraulic expansion caused by coolant temperature swings. Thus, pressuremay be maintained substantially constant at the pump 256 suction bycontrolling level and pressure of the surge drum.

The total coolant circulation flow rate through the coolant system andthe reactor jackets is typically maintained constant and may be measuredat flow element 258. The flow element 258 may represent, for example, aflow orifice plate installed in the coolant piping. A control system maycalculate the circulation flow rate based on the orifice size and themeasured upstream and downstream pressures. The flow rate indicationfrom flow element 258 may be received by flow controller 260, which maybe a control block in a distributed control system (DCS). To maintaintotal constant flow, the output of flow controller 260, using controlsignal 266, may adjust the position of the valve 262 on a flow bypassline 264. A well-known example of a distributed control system in theart is the Honeywell TDC-3000 control system. Normally, it is desirableto minimize the movement of valve 262 position to prevent cycling in thecoolant pump 26. Thus, additional means at other points in the systemmay assist in maintaining the total coolant circulation flow rateconstant.

During normal operation of a loop slurry reactor 210, heat is removedfrom the reactor contents, and heat is exchanged in cooler 268, whichmay represent one or more coolers. Heat is removed from the coolant incooler 268 to cool the coolant supply 252 to the reactor jackets 212A-H.The cooler 250 may be, for example, a shell and tube heat exchanger or aplate and frame heat exchanger. A cooling medium, such as cooling towerwater or sea water, flows through the cooler opposite the coolant,removing heat through the heat transfer surface area but not cominglingwith the coolant. The cooling medium flow is represented in this exampleby cooling water supply 272 and cooling water return 274. A coolingtower (not shown) may process the circulating cooling medium by removingheat from the cooling water return 272 and providing cooled coolingwater supply 274. Thus, the cooling tower water removes heat from thecoolant, which in turn removes heat from the reactor 210. In oneexample, the cooler 268 represents four plate and frame exchangercoolers that operate in parallel, each cooler having approximately 200stainless steel (304) plates and approximately 1600 square feet of heattransfer surface, with the heat transfer coefficient varying from about200 to over 800 Btu/hr/sq. ft/° F. as a function of coolant flow rate.Heat removed is about 15.5 million Btu/hr removed per cooler with adesign pressure drop of approximately 3 psi on the coolant side. For thetemperature control, coolant controller 276 (coolant temperaturecontroller) maintains the temperature of the coolant supply to thereactor jacket. Coolant controller 276 sends an output signal 278 toadjust the positions of valve 270 (and potentially other valves).

C. Reduced Coolant Flow

As mentioned, the flow rate of coolant through the reactor jackets212A-H may be reduced by allowing a greater temperature rise of thecoolant. Traditionally, the temperature difference, ΔT, between thecoolant supply 252 and the coolant return 254 has been maintained at 10°F. In other words, the temperature of the coolant return 254 has beenallowed to increase only to a temperature of about 10° F. greater thanthe temperature of the coolant supply 252. Currently, however, thetemperature of the coolant return 254 is now allowed to increase greaterthan 10° F. (e.g., 15-45° F. and higher) than the temperature of thecoolant supply 252. Thus, the cooling circuit may absorb the same ormore amount of heat from the reactor 210 with less coolant flow throughthe reactor jackets. For the example of a ΔT of 20° F. relative to thecustomary ΔT of 10° F., the flow rate is cut in about half. Thus thesize of the coolant pump 256 motor may reduced (e.g., from 1250horsepower to 600 horsepower).

VIII. Guide Vanes for the Loop Reactor Pump

The present techniques provide for use of guide vanes in the loopreactor pump that circulates the contents of the reactor. Such use ofguide vanes improves pump efficiency, reduces electrical consumption,and decrease normalized electrical usage by increasing polyolefinproduction rate. In addition to improved pump efficiency, implementationof the guide vanes improves several performance characteristics of theloop reactor and loop reactor pump. Gains occur, for example, incirculation rate, pump differential pressure, expected solids operatingcapability in the loop reactor, and as mentioned, the polyolefinproduction rate in the reactor, and so on. In the case of a 24-inchouter diameter (OD) loop reactor, use of guide vanes in the reactor pumpmay provide for polyolefin production in the range of 1.1 to 1.3 billionpound of polyolefin per year. Guide vanes may be employed in newinstallations or in the retrofit of existing loop reactor pumps toproduce higher pump head and slurry velocity, which enables the highersolids level and residence time in the reactor. Increases in pump headmay range, for example, from 5 to 25%.

Guide vanes may be utilized, for example, on loop pumps having a nominalOD in the range of 24 inches to 32 inches. Exemplary conditions of suchpumps are 240-300 feet of heat at 35,000-40,000 gallons per minute (gpm)with a pumping efficiency improvement in the range of 1-4%. The guidevanes allow for a larger reactor having the same circulation rate as asmaller reactor. In the specific example of a 30 inch pump, the pumphaving guide vanes provides adequate circulation in a 45,000-55,000gallon reactor. Other configurations for even larger increases inreactor volume may include larger pump diameters, use of two reactorpumps, or increasing the reactor diameter to shorten the reactor length,and so forth.

In general, three to six guide vanes may be employed, having a relativevane angle in the range of 0 to 30 degrees. As will be appreciated bythose of ordinary skill in the art, the relative vane angle is the angleof the guide vane relative to the leading edge angle of the pumppropeller. In other words, the relative vane angle is the difference inthe average of the angle of the guide vane exit and lead angle of thepump blade relative to the plane of propeller rotation. A largerpositive number of the relative vane angle generally means that thereactor slurry is rotated less by the preswirl guide vanes, while asmaller or negative number generally means that the slurry is rotatedmore. The direction of preswirl rotation of the slurry is in theopposite direction of the pump propeller rotation.

A typical guide vane may be welded to the wall of the reactor pipeupstream of the pump propeller. Placement of the guide vanes may be from0.1 to 2 pipe diameters upstream of the pump propeller. The guide vanesmay be positioned clear of the propeller hub and upstream of the reactorflange that connects to the pump suction. In this case, disassembly ofthe pump suction pipe may be facilitated where the guide vanes do notextend downstream of the flange.

In one example, the guide vanes start at about 24 inches in length, 6-7inches tall, and 0.6-0.9 inches thick. The guide vanes may curved andbent so that the guide vanes are substantially parallel to the directionof flow and the discharge end is at the desired relative angle whilefollowing along the inside of the suction pipe of the pump. The upstreamedge of the guide vanes may be sloped so that if debris or large polymerfragments (e.g., polymer “rope” or “strings”) catches on the upstreamedge, the debris or fragments may tend to advantageously slide to thecenter of the pipe and then be free of the guide vanes.

IX Continuous Take Off of the Reactor Effluent Discharge

A. Configuration and Benefits

FIGS. 10-12 illustrate a continuous take-off mechanism of the reactordischarge 22. Referring to FIG. 10, a continuous takeoff mechanism 280disposed on a pipe elbow of the loop slurry reactor 210, is depicted.The continuous takeoff mechanism 280 includes a take-off cylinder 282, aslurry withdrawal line 284, an emergency shut-off valve 285,proportional motor valve 286 to regulate flow, and a flush line 287. Thereactor 210 may be operated “liquid” full, and because the reactorliquid contents are slightly compressible, pressure control of theliquid through the system may be accomplished with a valve. Further,where diluent input is held substantially constant, and the proportionalmotor valve 286 may be used to control the rate of continuous withdrawaland to maintain the total reactor pressure within designated set points.

Referring to FIG. 11, which is taken along section line 11-11 of FIG.10, a smooth-curved pipe elbow having the continuous take-off mechanism280, is depicted. Thus the illustrated pipe elbow may be considered amappendage-carrying elbow. As shown, the mechanism includes take-offcylinder 282 which is attached, in this instance, at a right angle to atangent to the outer surface of the elbow. Further, coupling to thecylinder 282 is the slurry withdrawal line 284. Disposed within the takeoff cylinder 282 is a ram valve 288, which may serve at least twopurposes. First, it may provide a clean-out mechanism for the take-offcylinder if it should ever become fouled with polymer, for example.Second, it may serve as a shut-off valve for the entire continuoustake-off assembly.

FIG. 12 shows an attachment orientation for the take-off cylinder 282which is affixed tangentially to the curvature of the elbow and at apoint just prior to the slurry flow turning upward. The opening may beelliptical to the inside surface, for example, and further enlargementmay be implemented to improve solids take-off. Finally, it should benoted that a variety of orientations of the attachment of the take-offcylinder 282 may be implemented.

A continuous take-off of product slurry of an olefin polymerizationreaction carried out a loop reactor in the presence of an inert diluentallows operation of the reactor at a much higher solids concentrationthan with the conventional settling leg(s) used to discharge thepolymer. For example, production of predominantly ethylene polymers(polyethylene) in isobutane diluent has generally been limited to amaximum solids concentration in the reactor of 37-40 weight percent (wt.%) with the settling leg configuration. However, the continuous take-off(CTO) has been found to allow significant increases in solidsconcentration. As a result, solids concentration of greater than 50 wt.% in the reactor may implemented with the continuous takeoff. It shouldbe emphasized that in a commercial operation, as little as a onepercentage point increase in solids concentration is of majorsignificance. Such an increase, for example, allows higher productionrates of polyethylene, and thus generally gives increased normalizedenergy efficiency. Furthermore, less liquid in the reactor discharge 22may place less of a load on downstream recovery and fractionationsystems 24 and 30, thus reducing downstream energy consumption.Additionally, this technique may present savings in electricalconsumption because the continuous take-off discharge removes more finesfrom the reactor than the conventional discharge. With less surface areaof particles in the reactor, the fluid mixture may operate at a lowerviscosity, providing for easier circulation of the mixture through thereactor, and thus, less demanding pumping and associated horsepowerrequirements.

B. Polyolefin Particle Size

Furthermore, increasing the solids carrying capacity of the reactor alsoincreases the capability to operate the reactor at higher space-timeyield (e.g., a desired 2.6 or greater) as measured in pounds of polymerproduct produced per hour for each gallon of reactor volume orequivalent measures. Such an increase in the space-time yield inconjunction with a reduced incidence of reactor fouls may result inincreased polyolefin production and throughput at the reactor 10.

To increase the solids carrying capacity of the reactor, it may bedesirable to produce polymer particles in a desired size range such thatthe polymer particles are more likely to remain suspended, therebyallowing a greater weight percentage of solids to be achieved in thereactor. For example, an Englehard Lynx 100 catalyst, which on averageproduces smaller polymer particles than those produced using a Davidson969 MS Chrome catalyst, may be used to achieve a higher solids level ina reactor without inducing a foul. In this example, the polymerparticles produced by the Lynx 100 catalyst may be circulated at highersolids levels than comparable polymer particles produced by the 969 MScatalyst.

The desired size range may vary depending on the polymer product andreaction conditions. In one embodiment, to maintain suitable slurryconditions in a loop slurry reactor running under reaction conditionssuch as those discussed with regard to FIG. 1, less than 1% by weight ofthe polymer particles are greater than 1,500μ across. In anotherembodiment, less than 5% by weight of the polymer particles are greaterthan 1000μ across. In yet another embodiment, for less than 0.1% byweight of the polymer particles are greater than 1,500μ across and/orfor less than 0.5% by weight of the polymer particles are greater than1000μ, across

At the other extreme, to avoid problems associated with excessivenumbers of fine particles, in one embodiment, less than 5% by weight ofthe polymer particles are less than 100μ across and, in anotherembodiment, less than 0.5% by weight of the polymer particles are lessthan 100μ across. Furthermore, in yet another embodiment, more than 70%by weight of the polymer particles are between 300μ and 500μ across and,in an additional embodiment, more than 80% by weight of the polymerparticles are between 300μ and 500μ across. In yet another embodiment,more than 90% by weight of the polymer particles are between 300μ and500μ across.

Production of polymer particles having size distributions in accordancewith these preferences may be accomplished by a variety of techniques.For example, a catalyst may be employed which, due to the catalyst size,shape, reactive surface area, or other catalyst activity characteristic,produces polymer particles in the desired size range. In particular, thesize of the polymer particles produced by a catalyst generally variesproportionally with the catalyst particle size; that is, smallercatalysts generally produce smaller polymer particles. The weightpercentage of different sized polymer particles may vary betweencatalysts and generally corresponds to the catalyst particle size. Forinstance, a 25μ EP30X catalyst does not produce measurable amounts ofthe polymer particles larger than 1190μ, unlike the larger catalysts.Similarly, the catalysts smaller than 100μ produce less than 5% byweight of polymer particles greater than 1,000μ across while 100μcatalysts produce more than 5% by weight of polymer particles greaterthan 1,000μ across. While catalyst size may be one factor whichdetermines polymer particle size, other factors, such as morphology,active site accessibility, and so forth, may also contribute to therange of polymer particle sizes produced by a given catalyst.

X. Fractionation System

A. Diluent Purification

A purpose of the fractionation system 30 in polyolefin production is topurify the diluent discharged from the reactor system (e.g., from loopslurry reactor 210) and which is flashed/recovered in thediluent/monomer recovery subsystem 24. Initially, however, it should benoted, again, that the flashed diluent from the diluent/monomer recoverysystem 24 may instead be condensed and passed through a treater, such asa molecular sieve system, and directly recycled to the loop slurryreactor 210, bypassing the fractionation system 30. The treater mayremove undesirable components, such as the catalyst poison (e.g., water)injected upstream of the high pressure flash chamber 44 in the reactordischarge 22.

On the other hand, some or all of the recovered diluent from thediluent/monomer recovery system 24 may be sent through fractionationcolumns in the fractionation system 30 to remove heavy components, suchas hexene, hexane, and oligomers. The columns may also remove lightcomponents, such as ethane that enters with the ethylene feedstock,nitrogen from the purge column 228, unreacted ethylene from the reactor210, and so forth. In one arrangement, the fractionation subsysteminitially removes heavy components in a heavies column (also calleddiluent recycle column, recycle isobutane column, and the like) and thenremoves lighter components in a subsequent lights column (also calleddiluent purification column, isobutane purification column, and thelike).

B. Heavies Column

To remove heavy components, the first column (heavies column or diluentrecycle column) may discharge heavy components (e.g., hexene, hexane,and oligomers) out the bottom of the column to the flare. In certainconfigurations, the first column may also produce a side stream ofdiluent product (e.g., isobutane) that typically contains a measurableamount of light components (e.g., ethylene) but is acceptably recycledto the loop slurry reactor 210. In older configurations, this diluentproduct stream recycled to the reactor 210 may comprise the bulk of therecovered diluent received by the fractionation system 30 from thediluent/monomer recovery subsystem 24. The first column may also producean overhead lights stream comprising primarily diluent, inertcomponents, and ethylene, which may be partially condensed.Non-condensed components (e.g., nitrogen, ethylene) may be flared orrecycled to the supplier, or may be vented as feed to the downstreamsecond (lights) column. Condensed components of the overhead stream maybe used as reflux to the first column and as reflux or feed to thesecond column, depending on the configuration employed at the particularpolyolefin facility.

C. Lights Column

To remove light components, the second column (lights or diluentpurification column), removes light components (e.g., ethylene, ethane,and nitrogen) to give a more pure diluent product which may besubstantially olefin-free (with the heavy components already removed inthe upstream column). The second column typically processes a smalleramount of diluent than the first column. The small stream ofmonomer-free (olefin-free) diluent may exit the bottom of the secondcolumn and be used in catalyst preparation, catalyst delivery, catalystflushes, reactor flushes where catalyst is present, and so forth. Theavailability of monomer-free diluent is beneficial for thesecatalyst-related functions because it may be important that olefinmonomer not come into contact with catalyst outside of the reactor 210.Such contact could result in polymerization in undesirable parts of theprocess, which may plug equipment, cause operability problems, expendcatalyst, and so forth.

D. Fractionation System Equipment and Process

Referring to FIG. 13, a process flow diagram of the fractionation system30 is depicted. The heavies column 290 and the lights column 292, eachhaving appropriate internals 294 and 296 (e.g., packing, distillationtrays, etc.), are illustrated. A flash gas stream, or in thisillustration, the hydrocarbon stream 240 (primarily diluent) from thepurge column 228 is fed to the lights column 290, which may operatetypically at a pressure in the range of 125-175 psig and at atemperature in the range of 140-350° F. The lights column 290 separatesunreacted monomer (e.g., ethylene) and lighter components (e.g., ethane,propane, nitrogen, etc.) in the overhead, as well as the heaviercomponents such as hexane-1 and oligomers, from the diluent (e.g.,isobutane) in the bottoms discharge. The overhead 298 from column 290may be partially condensed in a condenser 300, such as a heat exchangerthat utilizes a cooling medium (e.g., cooling tower water). Furthermore,it should be noted that fresh diluent 302 may added to the circuitdownstream of the condenser 300.

The uncondensed vapors 304 may be separated in the accumulator 306 andfed to the lights column 292. In an alternate configuration, all or aportion of the vapors 304 may be vented to upstream supplier operations(e.g., olefin plant, petroleum refinery, etc.) or to the flare. Thecondensed liquid 308 from the accumulator 306 may be returned via pump310 as reflux 312 to the heavies column 290. The liquid 308 may also besent as reflux or feed 314 to the downstream lights column 292.Furthermore, the liquid 308, which is typically primarily diluent, maybe recycled to the reactor 210, as indicated by reference numeral 316(e.g., via a storage vessel and pump). Lastly, a steam reboiler 318(e.g., shell and tube heat exchanger) vaporizes the heavy components 320(e.g., hexene and oligomers) discharging from the bottom column 290,with a portion of the components 320 discharged to the flare.

The lights column 292 may receive condensed components 314 anduncondensed components 304, and separate a light component stream 322(e.g., nitrogen, ethane, ethylene) for recycle to the supplier, or as avent to the flare. At the bottom of the column 292, “olefin-free”diluent 324, which is substantially free of olefin monomer, dischargesfrom the column 292 and may be collected in an olefin-free diluent tank326, for example. The olefin-free diluent 82 may then be delivered viapump 328 (e.g., centrifugal pump, positive displacement pump, etc.) forreactor flushes and catalyst dilution (see FIG. 2). A steam reboiler 330vaporizes a portion of the liquid diluent 324 discharging from thebottom of the lights column 292 to provide a return vapor flow 332 tothe column 292. Furthermore, column 292 may be refluxed by arefrigerated condenser 334, with refrigerant 336 supplied from arefrigeration system 338. In the illustrated embodiment, therefrigeration system 338 also processes the refrigerant return 339. Anexemplary refrigerant used is liquid propylene. The overhead operatingtemperature of the column 292 in one example, is in the range −10° F. to0° F., and the bottoms operating temperature is in the range of 145 to170° F.

Finally, as discussed, with direct recycle of 80 to 95 wt. % of thediluent and unreacted monomer recovered from the in the monomer/recoverysystem 24 to the feed and reactor systems 16 and 20. For example, flashgas 226 (FIG. 7) which discharges from the flash chamber 224 overhead,and which generally corresponds to the recycle stream 34 of FIG. 1, maybe sent as the recycle diluent 54 stream (FIG. 2) directly to thereactor 210 via the surge tank 68. Such direct recycle significantlyreduces the load on the fractionation system, including the load on theheavies column 290 and lights column 92. Thus, these columns (andsimilar fractionation columns) and associated steam reboiliers 318 and330 may be significantly reduced in size (e.g., 5-20% of theconventional size) for the same capacity polyolefin plant. Thus steamusage is significantly reduced and substantial energy is saved byemploying smaller columns.

IX. Extrusion/Loadout System

Referring to FIG. 14, a process flow diagram of the extrusion/loadoutsystem 36 of FIG. 1 is depicted. Polyolefin fluff 232 from the purgecolumn 228 (FIG. 7) may be pneumatically transferred, for example, usinga dilute phase blower, through a valve 340 in the extruder/loadoutsystem 36, and either into conduit 342 to the fluff silo 344, or intoconduit 346 to the extruder feed tank 348. The fluff silo 344 may beused to provide surge capacity during shutdown of the extruder (or ofother operations) in the extrusion/loadout system 36. On the other hand,the fluff silo 344 may also accumulate fluff to allow for full-rateoperation of the extruder while the upstream polymerization reactor 210“catches up” during start up of the reactor 210. The polyolefin fluff insilo 344 may be pneumatically transferred to the extruder feed tankthrough rotary valve 350 with the aid of a blower system.

Typically, however, the primary flow of polyolefin fluff 232 (which maygenerally correspond to fluff 28 of FIG. 1) is to the extruder feed tank348 via conduit 346. Downstream, rotary valve 352 may feed polyolefinfluff 354 to the extruder 356, where the extruder heats, melts, andpressurizes the polyolefin fluff 354. As will be appreciated by those ofordinary skill in the art, the fluff 354 from the extruder feed tank 348may be metered to the extruder 356 with a variety of meters, such as asmart flowmeter-type, master-feeder type, and so forth. Furthermore,additives may be injected into the fluff 354 stream at an addition ratewhich may be based on a specified ratio to the mass flow rate of thefluff 354. This ratio or “slave” feed of additives to fluff 354 may bespecified at a value to generate a desired recipe, for example, for eachpolyolefin grade or product, and to give the desired properties of thedownstream polyolefin pellets. Furthermore, the additive addition may beaccomplished with a liquid additive system, loss-in-weight-feeders, andthe like. In certain embodiments, one or more of lost-in-weight feedersmay be used to meter a pre-mixed additive package fed from a bulkcontainer, for example, from an extruder feed hopper to the extruder 356via the fluff 354 stream, and so on.

In general, the extruder 356 may melt, homogenize, and pump thepolyolefin polymer and additives through a pelletizer 358, which mayinclude a screen pack and heated die head, for example, which pelletizesthe mixture of fluff and additives. Further, pelletizer knife blades(i.e., under water) may cut the polyolefin melt extruded through the dieinto pellets. The pellets are typically quenched by water 368 and maytravel in a pellet-water slurry 360 from the pelletizer 358 to a pelletdewatering dryer 370. The dryer 370 may separate the free water 372 andthen dry the remaining surface water from the pellets by centrifugalforce. The dried pellets 374 may discharge onto a scalping screen 376,for example, which removes oversized and undersized pellets fromon-specification pellets.

Water 368 may be supplied to the pelletizer 358 from a water tank 362via a centrifugal pump 364 and cooler 366 (e.g., shell and tube heatexchanger). Water 372 removed from the pellet dryer 370 may return tothe water tank 362. The polyolefin pellets exiting the scalping screen376 may fall by gravity through a rotary valve 378 into a dense-phasepneumatic conveying line 380, for example, and may be transported topellet silos 386. The pellet silos may include storage tanks, blenders,off-specification storage tanks, and so on. In the illustratedembodiment, the blower package 382 provides nitrogen and/or air 384 toconvey the pellets via conveying line 380 to the pellet silos 386.Polyolefin pellets 388 may be loaded into rail cars 390, hopper cars,trucks, tote bins, bags, and so on. Pellets 388 may be loaded intohopper cars, for example, using a gravity type, air assisted,multiple-spout, loading system. Such a system may allow the hopper carto be automatically loaded at a rate higher than the polymerization andextrusion production rate. Thus, extra “time” generated by the higherloadout rates may be exploited to provide time to move the hopper carsor rail cars after filling, and to spot the next empty car.

A variety of energy proficient techniques may be implemented in theextrusion/loadout system 36. For example, as previously discussed, theupstream purge column 228 may be combined with the extruder feed tank348. Thus, the conveying system for transporting fluff 232, and thus theassociated electrical consumption of the blower in the conveying system,may be eliminated. Furthermore, in this example, the fluff 232 is warmer(e.g., 450° F.) than if experiencing the cooling effect (e.g., cooleddown to 80-100° F.) of nitrogen or air in a conveying system customarilyimplemented. With combining the purge column 228 into the extruder feedtank 348, the fluff 232 (becomes the fluff 354) and is fed to theextruder warmer than traditionally fed to the extruder 356. Therefore,less steam is consumed at the extruder to heat and melt the incomingfluff 354.

In general, the number of silos or storage vessels between thediluent/monomer recovery system 24 and the extrusion/loadout system 36may be reduced. In the illustrated embodiment, two vessels are depicted,a fluff silo 344 and an extruder feed tank 348. Traditionally, however,up to 10 to 15 storage vessels and associated blower conveying andrecycling/blending systems have been provided to store, blend, and feedfluff to the extruder 356. Thus, in the illustrated embodiment, lesselectricity is consumed relative to traditional operation of fluffstorage and feed.

Furthermore, the last remaining fluff silo 344 may be eliminated andthus, the associated blower package 351 and electrical consumption maybe eliminated. To do away with the fluff silo 344 and to give up theassociated fluff residence time, the wet end 42 may be more closelycoupled in operation with the dry end 44 (see FIG. 1). In other words,improved techniques may be implemented in the operation of thepolymerization reactor 210 in reactor system 20 (FIG. 1) to allow thereactor 210 to “back off” on production rate of polyolefin fluff, forexample, to accommodate upsets in the downstream extrusion/loadoutsystem 36 that normally may be accommodated by the surge capacity of afluff silo 344. For example, if the extruder 356 is shut downtemporarily, the polymerization reactor 210 may be subjected to a“mini-kill” or a “partial-kill,” where a relatively small portion (e.g.,part per billion range) of catalyst poison, such as carbon monoxide, isinjected into the reactor 210 to temporarily “kill” the polymerization.Thus, if a temporary shut down of the extruder 356 or other equipment inthe extrusion/loadout system 36 occurs, the discharge of polyolefinfluff 232 is temporarily stopped or reduced from the reactor 210discharge 22 due to lack of polymerization in the reactor 210.Therefore, the residence time in the extruder feed tank 348 may beadequate to retain the incoming fluff 232 until the extruder operationis restarted.

Finally, it should be noted that yet another blower conveying packagemay be eliminated in the extrusion/loadout system 36, and thus providingfor additional reduction in electrical consumption. The pellet waterpump 364 may be used to convey the pellets 374 into the pellet silos384, and thus, the blower package system 386 may be eliminated. Theelectrical consumption is significantly reduced because the typical sizeof the pellet water pump motor is only 25 horsepower, compared to ahorsepower ratings of the blower motor ranging from 250 to 500horsepower and higher. It should be noted that if pellet water pump 364is used to convey the pellet water slurry 360 up to above the silos 386,the pellet dryer 364 and scalping screen 376 may be relocated above thesilos 386, and thus allow for gravity-draining of the pellets 374 fromthe dyer 370 through the scalper 376 into the pellet silos 386.

X. Summary of Energy-Efficient Techniques

Some of the energy-efficiency techniques are itemized below.

-   -   A. In the polymerization reactor feed system, a mass flow meter,        instead of the conventional orifice plate meter, is used to        measure flow of monomer, eliminating the need to preheat the        monomer.    -   B. Further, a larger catalyst activator is employed, reducing        the amount of fuel gas consumed (combusted) to activate the        polymerization catalyst fed to the reactor.    -   C. Additionally, the number of treaters that remove catalyst        poisons from the reactor feed streams are reduced, providing for        more efficient scalability in regeneration of the treaters and        lower electrical consumption.    -   D. Moreover, an improved regeneration technique of the treaters        reduces the amount of inert components (e.g., nitrogen)        discharged to the flare header. This reduces the amount of fuel        gas (e.g., natural gas) injected into the flare header to        maintain an appropriate combustible content of the feed to the        flare.    -   E. In the reactor system itself, a continuous take off (CTO) of        the polyolefin slurry discharged from the reactor, instead of        the conventional intermittent discharge via a settling leg,        provides for a higher solids concentration in the reactor. A        larger concentration of polyolefin in the reactor may permit a        greater production rate of the polyolefin and thus reduce the        normalized consumption of energy, in part, by spreading fixed        energy costs over more pounds of polyolefin produced.        Furthermore, a continuous take-off discharge removes more fines        from the reactor than the conventional settling-leg discharges,        and thus with less surface area of particles in the reactor, the        fluid mixture operates at a lower viscosity providing for easier        circulation of the reactor contents. Therefore, the reactor        circulation pump may be downsized, utilizing less horsepower.    -   F. Additionally, a liquid phase reactor, such as a loop slurry        reactor, may be constructed of a material (e.g., high-strength        aluminum) having higher strength and thermal conductivity that        than steel, the traditional material used in the fabrication of        the loop slurry reactor. Such newer high-strength materials        provide for improved thinner reactor walls, improved        heat-transfer through the walls, and increased diameter of the        loop reactor, permitting a higher polyolefin production rate.    -   G. Another example in the reactor system is the use of guide        vanes in the reactor circulation pump, providing for increased        pumping efficiency (reduced electrical consumption) and        increased polyolefin production rate.    -   H. Yet another example in the reactor system is a technique        which specifies a greater increase in the temperature (e.g.,        from the traditional 10° F. to 15-45° F. and higher) of the        coolant flowing through the reactor jackets. Such increased        temperature difference between the coolant supply and return        imparts the same heat removal capability at lower flow rates of        coolant. Therefore, the coolant circulating pump may be sized        lower, requiring less horsepower.    -   I. In the diluent/monomer recovery system that processes the        effluent discharged from the polymerization reactor, savings in        electricity are accomplished by eliminating a low-pressure flash        of the diluent and the associated recycle compression.    -   J. Further savings may be acquired by eliminating the purge        column which removes residual hydrocarbon from the polyolefin        fluff particles. The hydrocarbon removal operation is instead        performed at the downstream extruder feed tank in the        extrusion/loadout system. This improvement allows for        utilization of the process pressure in an upstream flash        chamber, instead of a blower conveying system which consumes        electricity, to transport the polyolefin particles to the        extruder feed tank. The improvement also provides for warmer        polyolefin fluff particles (e.g., 450° F. versus 80-100° F. of        the fluff in the conventional conveying system) fed to the        downstream extruder, reducing the energy load on the extruder.    -   K. Furthermore the number of polyolefin fluff silos intermediate        the diluent/monomer recovery system and the extrusion/loadout        system are reduced in number via, in part, improved operation of        the upstream polymerization reactor and the downstream extruder.        Such reduction in silos or storage vessels reduced the number of        associated blowers and their electrical consumption.    -   L. In the extrusion/loadout system, electricity is saved via use        of a pellet water pump is to transport polyolefin pellets        discharging the extruder/pelletizer to the pellet silos instead        of the conventional blower conveying package. Indeed, the        horsepower requirement for the pellet water pump is an order of        magnitude lower than that of a pneumatic conveying blower.    -   M. In the fractionation system that processes the recovered        unreacted monomer and diluent from the polymerization reactor        and diluent/monomer recovery system, steam usage is reduced by        as much as 90 percent. Such reduction is afforded by direct        recycle of the diluent and monomer to the polymerization        reactor, bypassing the fractionation system, and thus allowing        for smaller fractionation columns and the associated steam        reboiler heat-exchangers.

1. A method for operating a polyolefin manufacturing process,comprising: feeding a monomer, a diluent, and a catalyst to apolymerization reactor; polymerizing the monomer in the polymerizationreactor to form polyolefin particles; discharging a slurry from thepolymerization reactor, wherein the slurry comprises monomer, diluent,and polyolefin particles; recovering polyolefin particles from theslurry by separating at least a majority of the diluent from the slurry;recycling a first portion of the separated diluent to the polymerizationreactor without fractionating the first portion; fractionating a secondportion of the separated diluent to provide diluent substantially freeof monomer; extruding and pelletizing the recovered polyolefin particlesto form polyolefin pellets; transporting polyolefin pellets to aload-out area; and consuming less than 445 kilowatt-hours of energy permetric ton of polyolefin produced based on consumption of electricity,steam, and fuel gas.
 2. The method of claim 1, comprising producing atleast 600 million pounds of polyolefin pellets per year.
 3. A method foroperating a polyolefin manufacturing process, comprising: feeding amonomer, a diluent, and a catalyst to a polymerization reactor;polymerizing the monomer in the polymerization reactor to formpolyolefin particles; discharging a slurry from the polymerizationreactor, wherein the slurry comprises monomer, diluent, and polyolefinparticles; recovering polyolefin particles from the slurry by separatingat least a majority of the diluent from the slurry; recycling a firstportion of the separated diluent to the polymerization reactor withoutfractionating the first portion; fractionating a second portion of theseparated diluent to provide diluent substantially free of monomer;extruding and pelletizing the recovered polyolefin particles to formpolyolefin pellets; transporting polyolefin pellets to a load-out area;and consuming less than 325 kilowatt-hours of electricity per metric tonof polyolefin pellets produced.
 4. The method of claim 3, comprisingoperating a feed treater as a spare for both removing catalyst poisonsin monomer fed to the polymerization reactor and for removing catalystpoisons in diluent fed to the polymerization reactor.
 5. The method ofclaim 3, comprising circulating a coolant through a jacket of thepolymerization reactor and maintaining a temperature increase of thecoolant through the jacket in the range of 15° F. to 45° F.
 6. Themethod of claim 3, wherein discharging a slurry from the polymerizationreactor comprises substantially continuously discharging the slurry fromthe polymerization reactor.
 7. The method of claim 3, wherein separatingdiluent from the slurry comprises flashing diluent from the slurry andcondensing the flashed diluent without compression.
 8. The method ofclaim 3, wherein separating the diluent and separating the polyolefinparticles comprise: subjecting the slurry to a high-pressure flash togenerate a flash stream comprising diluent and a solids streamcomprising polyolefin particles and residual diluent; and purging thesolids stream to remove residual diluent from the polyolefin particles,wherein the solids stream is not subjected to an intermediatelow-pressure flash.
 9. The method of claim 3, comprising transportingthe polyolefin particles separated from the slurry to an extruder feedtank without substantial intermediate hold-up of the transportedpolyolefin particles.
 10. The method of claim 3, wherein transportingpolyolefin pellets to a load-out area comprising transporting thepolyolefin pellets to a pellet silo via a pellet water pump disposed ata discharge of an upstream extruder/pelletizer.
 11. A method foroperating a polyolefin manufacturing process, comprising: feeding amonomer, a diluent, and a catalyst to a polymerization reactor;polymerizing the monomer in the polymerization reactor to formpolyolefin particles; discharging a slurry from the polymerizationreactor, wherein the slurry comprises monomer, diluent, and polyolefinparticles; recovering polyolefin particles from the slurry by separatingat least a majority of the diluent from the slurry; recycling a firstportion of the separated diluent to the polymerization reactor withoutfractionating the first portion; fractionating a second portion of theseparated diluent to provide diluent substantially free of monomer;extruding and pelletizing the recovered polyolefin particles to formpolyolefin pellets; transporting polyolefin pellets to a load-out area;and consuming less than 144 kilograms of steam per metric ton ofpolyolefin pellets produced.
 12. The method of claim 11, comprisingmeasuring a flow rate of ethylene monomer fed to the polymerizationreactor with a mass meter.
 13. The method of claim 11, wherein the firstportion of separated diluent comprises at least 80 weight % of thediluent discharge in the slurry from the polymerization reactor.
 14. Amethod for operating a polyolefin manufacturing process, comprising:feeding a monomer, a diluent, and a catalyst to a polymerizationreactor; polymerizing the monomer in the polymerization reactor to formpolyolefin particles; discharging a slurry from the polymerizationreactor, wherein the slurry comprises monomer, diluent, and polyolefinparticles; recovering polyolefin particles from the slurry by separatingat least a majority of the diluent from the slurry; recycling a firstportion of the separated diluent to the polymerization reactor withoutfractionating the first portion; fractionating a second portion of theseparated diluent to provide diluent substantially free of monomer;extruding and pelletizing the recovered polyolefin particles to formpolyolefin pellets; transporting polyolefin pellets to a load-out area;and consuming less than 2.8 kilograms of fuel gas per metric ton ofpolyolefin pellets produced.
 15. The method of claim 14, comprisingactivating the catalyst in a catalyst activator prior to feeding thecatalyst to the polymerization reactor, wherein the catalyst activatorcomprises an inner vessel having a nominal inner diameter in the rangeof 48 inches to 72 inches.
 16. The method of claim 14, wherein feedingdiluent to the polymerization reactor comprises removing catalystpoisons from the diluent in a feed treater.
 17. The method of claim 16,comprising regenerating the feed treater with nitrogen and dischargingsubstantially-clean nitrogen to the atmosphere from the feed treaterduring the regeneration.
 18. A method for operating a polyolefinmanufacturing process, comprising: feeding a monomer, a diluent, and acatalyst to a polymerization reactor; polymerizing the monomer in thepolymerization reactor to form polyolefin particles; discharging aslurry from the polymerization reactor, wherein the slurry comprisesmonomer, diluent, and polyolefin particles; recovering polyolefinparticles from the slurry by separating at least a majority of thediluent from the slurry; recycling a first portion of the separateddiluent to the polymerization reactor without fractionating the firstportion; fractionating a second portion of the separated diluent toprovide diluent substantially free of monomer; extruding and pelletizingthe recovered polyolefin particles to form polyolefin pellets;transporting polyolefin pellets to a load-out area; and maintaininglosses of nitrogen in the polyolefin manufacturing system at less than26 normal cubic meters of nitrogen per metric ton of polyolefin pelletsproduced.
 19. A method for operating a polyolefin manufacturing process,comprising: feeding a monomer, a diluent, and a catalyst to apolymerization reactor, wherein the diluent comprises isobutane;polymerizing the monomer in the polymerization reactor to formpolyolefin particles; discharging a slurry from the polymerizationreactor, wherein the slurry comprises monomer, diluent, and polyolefinparticles; recovering polyolefin particles from the slurry by separatingat least a majority of the diluent from the slurry; recycling a firstportion of the separated diluent to the polymerization reactor withoutfractionating the first portion; fractionating a second portion of theseparated diluent to provide diluent substantially free of monomer;extruding and pelletizing the recovered polyolefin particles to formpolyolefin pellets; transporting polyolefin pellets to a load-out area;and maintaining losses of the isobutane in the polyolefin manufacturingsystem at less than 1.7 kilograms of isobutane per metric ton ofpolyolefin pellets produced.